Process for production of water absorbent resin

ABSTRACT

A water-absorbent resin is produced by dispersing a solid blowing agent having an average particle diameter within a range of from 1 μm to 100 μm in an aqueous monomer solution containing an unsaturated monomer and a cross-linking agent, and then polymerizing the unsaturated monomer. The water-absorbent resin has excellent water absorption characteristics, such as improved dispersion and absorption rate of aqueous fluid, enhanced water retention capacity and dry touch, lower water-soluble component content, and lower residual monomer content. When a water-absorbent resin composition using the water-absorbent resin is used for, for example, a sanitary material, it is possible to improve the absorption rate and water retention capacity, and prevent leakage of fluid from the sanitary material.

This application is a division of application Ser. No. 08/687,377, filedAug. 2, 1996 U.S. Pat. No. 5,985,944.

FIELD OF THE INVENTION

The present invention relates to water-absorbent resins suitable for usein absorbent articles, for example, sanitary materials (body fluidabsorbent articles) such as paper diapers (disposable diapers), sanitarynapkins, so-called incontinence pads (articles for incontinent person),wound protecting material and wound healing material, building material,water retentive material for soil, drip absorbing and freshnessretentive materials for food, and waterproof material. The presentinvention also relates to a process for producing such water-absorbentresins, and water-absorbent resin compositions using the water-absorbentresins.

BACKGROUND OF THE INVENTION

In recent years, water-absorbent resins for absorbing body fluids suchas urine, sweat, and blood are widely used as a constituent element ofsanitary materials such as paper diapers, sanitary napkins, incontinencepads, wound protecting material, and wound healing material. Suchwater-absorbent resins are utilized not only as sanitary materials, butalso applied to various uses to absorb and retain water and absorbmoisture, for example, building material, water retentive material forsoil, drip absorbing and freshness retentive materials for food, andwaterproof material.

Known water absorbing resins include partially neutralized andcross-linked acrylic acid (Japanese Publication for Unexamined PatentApplications No. (Tokukaisho) 55-84304, 55-108407, and 55-133413, andU.S. Pat. No. 4,654,039), hydrolyzed starch-acrylonitrile graft polymer(Japanese Publication for Examined Patent Application No. (Tokukosho)49-43995, neutralized starch-acrylic acid graft polymer (JapanesePublication for Unexamined Patent Application No. (Tokukaisho)51-125468), saponified vinyl acetate-acrylate copolymer (JapanesePublication for Unexamined Patent Application No. (Tokukaisho)52-14689), hydrolyzed acrylonitrile copolymer or acrylamide copolymer,or cross-linked acrylonitrile copolymer and acrylamide copolymer(Japanese Publication for Unexamined Patent Application No. (Tokukaisho)53-15959), cross-linked carboxymethyl cellulose, and cross-linkedcationic monomer (Japanese Publication for Unexamined PatentApplications No. (Tokukaisho) 58-154709 and 58-154710), cross-linkedisobutylene-maleic anhydride copolymer material, cross-linked copolymerof 2-acrylamido-2-methylpropane sulfonic acid and acrylic acid,cross-linked polyethyleneoxide, and cross-linked copolymer ofmethoxypolyethylene glycol and acrylic acid.

All of the water-absorbent resins are in the form of particles or powderhaving a particle diameter of around 0.01 mm to 5 mm. The absorptionrate of the water-absorbent resin is generally determined by theparticle diameter. There is a tendency that the absorption rate of eachparticle increases as the particle diameter becomes smaller (“Polymers”Vol. 36, page 614, Polymer Association, 1987).

However, in actual, as the particle diameter becomes smaller, the liquidpermeability for allowing aqueous fluids, for example, body fluids, toflow between the particles, is lowered. Namely, a so-called gel blockingphenomenon occurs. Therefore, when using the water-absorbent resin, itis necessary to select an optimum particle diameter by considering theabsorption rate and the liquid permeability. The tendency of causing agel blocking phenomenon becomes higher as the absorption rate of thewater-absorbent resin increases. The main causes of the gel blockingphenomenon are a decrease in the void space and an increase in tackbetween particles after being swelled, under pressure.

In order to improve the water absorption characteristics of thewater-absorbent resin, particularly, the absorption rate, variousproduction methods and modification methods of water-absorbent resinshave been proposed as shown below. More specifically, as the productionand modification methods of water-absorbent resins, for example, thefollowing two methods have been proposed. {circle around (1)}Application of secondary cross-linking treatment, i.e., improving thecross-link density in the vicinity of a particle surface. {circle around(2)} Increasing the particle surface area by granulation, foaming,formation of pores, or the like.

The method {circle around (1)} includes methods which use the followingmaterials as a surface cross-linking agent. Namely, a method usingpolyhydric alcohol; a method using a polyglycidyl compound, apolyaziridine compound, a polyamine compound and a polyisocyanatecompound; a method using glyoxal; a method using polyvalent metal salt;a method using a silane coupling agent; a method using a mono epoxycompound; a method using a polymer containing an epoxy group; a methodusing an epoxy compound and a hydroxy compound; and a method usingalkylene carbonate.

For instance, the following methods were also proposed. A method inwhich a cross-linking reaction is performed under the presence ofinactive inorganic powder (U.S. Pat. No. 4,587,308). A method in which across-linking reaction is performed under the presence of dihydricalcohol. A method in which a cross-linking reaction is performed underthe presence of water and an ether compound. A method in which across-linking reaction is performed under the presence of alkylene oxideadded monohydric alcohol, organic acid salt, or lactam. A method inwhich more than one kind of cross linking agents having differentsolubility parameters are used. Moreover, the methods for improving thecross-link density in the vicinity of the particle surface are disclosedin U.S. Pat. No. 4,666,983, No. 5,140,076 and No. 5,229,466, andJapanese Publication for Unexamined Patent Applications No. (Tokukaisho)59-62665 and No. (Tokukaihei) 5-508425.

As the method {circle around (2)}, for example, a method using a blowingagent during polymerization or cross-linking was proposed. The methodusing a blowing agent includes, for example, methods in which across-linked structure is introduced into a linear water-soluble polymerwhile performing neutralization using a blowing agent such as carbonate(U.S. Pat. No. 4529,739 and No. 4,649,164), methods in which carbonatesalt is added to monomers (Japanese Publication For Examined PatentApplications No. (Tokukosho) 62-34042, No. (Tokukohei) 2-60681, and No.(Tokukohei) 2-54362, and U.S. Pat. No. 5,118,719, No. 5,154,713 and No.5,314,420), a method in which monomers are polymerized using a microwaveunder the presence of carbonate salt (U.S. Pat. No. 4,808,637), methodsin which an organic solvent having a boiling point within a range offrom 40° C. to 150° C. is added to a specified monomer and thenpolymerized (Japanese Publication For Unexamined Patent Application No.(Tokukaisho) 59-18712, and U.S. Pat. Nos. 4,552,938, No. 4,654,393 andNo. 4,703,067), and methods in which a hydrophobic organic solvent isadded and polymerized under specified pressure (U.S. Pat. No. 5,328,935and No. 5,338,766). Additionally, methods in which a blowing agent isadded after polymerizing monomers were also proposed (JapanesePublication For Unexamined Patent Applications No. (Tokukaisho)56-13906, No. (Tokukaisho) 57-182331, and No. (Tokukaisho) 57-208236).

Furthermore, the following methods were also proposed. A method in whicha polarity is given to particles using a microwave (WO No. 91/02552).Methods in which fine particles are made into secondary particles bygranulation (WO No. 93/24153, U.S. Pat. No. 5,002,986, No. 5,300,565,No. 5,140,076 and No. 4,732,968).

With the use of the methods {circle around (1)} and {circle around (2)},it is possible to improve the absorption rate of the water-absorbentresin to some extent.

However, the water-absorbent resin prepared by cross-linking cannotachieve a high absorption rate which is required when it is used, forexample, in sanitary materials. In addition, the water-absorbent resinprepared by cross-linking while foaming a linear polymer does not havesufficient absorbent capacity (water retention capacity), and requires ahigh cost. Whereas the porous water-absorbent resin prepared by foamingwhile polymerizing monomers is excellent in terms of the absorption rateand cost. However, it is difficult to control the timing of foaming, andcannot achieve a uniform pore diameter. Thus, these water-absorbentresins failed to sufficiently improve various characteristics related tothe dispersion of aqueous fluid, water-soluble component content,residual monomer content, and dry touch (these characteristics will beexplained later).

More specifically, the water-absorbent resins obtained by theabove-mentioned production method or modification method have suchdisadvantage that the mutual balance of conflicting characteristics,such as the dispersion of aqueous fluid, the water-soluble componentcontent, and dry touch, is not satisfactory. Namely, the above-mentionedconventional water-absorbent resins do not have sufficiently improvedwater absorption characteristics, and cannot provide high waterabsorption characteristics which are required when used, for example, insanitary materials.

An object of the production method and modification method is to producea water-absorbent resin capable of promptly absorbing aqueous fluid whenthe water-absorbent resin comes into contact with the aqueous fluid.Therefore, these methods are designed without substantially consideringthe water absorption characteristics that are required of thewater-absorbent resin when the water-absorbent resin is used in asanitary material, particularly, when a large amount of water-absorbentresin is used in sanitary material to reduce the thickness of thesanitary material.

In the sanitary material using a large volume of the water-absorbentresin, it is necessary to improve the absorption. However, if theabsorption rate is increased, the gel blocking phenomenon tends tooccur. In order to reduce the incidence of the gel blocking phenomenon,for example, an attempt has been made to improve the elasticity of gel.However, if the elasticity of gel is improved, the water retentioncapacity of the water-absorbent resin is lowered. Therefore, even if thewater-absorbent resin having improved absorption rate and elasticity ofgel is used in the sanitary material, it is hard to say that thesanitary material is prevented from leakage. Thus, there is a demand fora water-absorbent resin capable of keeping various characteristics suchas absorption rate and water retentive ability, and achieving improveddispersion of aqueous fluid between particles after absorption, that isa characteristic conflicting the above characteristics.

The present invention was carried out to solve the above conventionalproblems. An objective of the present invention is to provide awater-absorbent resin having excellent water absorption characteristicssuch as the dispersion and absorption rate of aqueous fluid, waterretentive ability and dry touch, lower water-soluble component content,and lower residual monomer content. Another objects of the presentinvention to provide a process for producing the water-absorbent resins,and a water-absorbent resin composition using the water-absorbent resin.

DETAILED DESCRIPTION OF THE INVENTION

In order to achieve the above objects, the present inventors fullystudied water-absorbent resins, process for producing thewater-absorbent resins, and water-absorbent resin compositions. It wasfound as a result of study that a water-absorbent resin obtained bydispersing a solid blowing agent in the form of particles having anaverage particle diameter within a range of from 1 μm to 100 μm in anaqueous monomer solution containing an unsaturated monomer and across-linking agent and then polymerizing the unsaturated monomer, hasexcellent water absorption characteristics, such as dispersion andabsorption rate of aqueous fluid, water retention capacity and drytouch, and lower water-soluble component content and lower residualmonomer content. It was also found as a result of study that, when awater-absorbent resin composition using the water-absorbent resin isapplied to, for example, a sanitary material, the absorption rate andthe water retention capacity are enhanced, thereby preventing thesanitary material from leakage. The present invention was completedbased on these findings.

Namely, in order to achieve the above objects, the process for producinga water-absorbent resin of the present invention is characterized bydispersing a solid blowing agent having an average particle diameterwithin a range of from 1 μm to 100 μm in an aqueous monomer solutioncontaining an unsaturated monomer and a cross-linking agent and thenpolymerizing the unsaturated monomer.

With this process, it is possible to industrially producewater-absorbent resins having excellent water absorptioncharacteristics, such as dispersion and absorption rate of aqueousfluid, water retention capacity and dry touch, and lower water-solublecomponent content and lower residual monomer content, in a simplifiedmanner at low costs.

Moreover, in order to achieve the above objects, the water-absorbentresin of the present invention is characterized by that it is porouswith an average pore diameter ranging from 10 μm to 500 μm, and has anabsorbent capacity of not lower than 25 g/g 60 minutes after theinitiation of water absorption under pressure, a water-soluble componentcontent of not higher than 15 weight percent, and a residual monomercontent of not higher than 500 ppm.

Furthermore, in order to achieve the above object, the water-absorbentresin composition of the present invention is characterized by that thewater retention capacity is not lower than 20 g/g, the absorption rateis not higher than 120 seconds, and the liquid permeability underpressure is not higher than 200 seconds.

This structure can provide a water-absorbent resin and a water-absorbentresin composition having excellent liquid permeability and dispersionunder pressure, without causing a gel blocking phenomenon, and improvedabsorption rate and absorbent capacity.

The following description will discuss the present invention in detail.

The unsaturated monomer used as a starting material in the presentinvention is soluble in water. Examples of the unsaturated monomer are:

monomers containing an acid group, such as acrylic acid,β-acryloyloxypropionic acid, methacrylic, acid, crotonic acid, maleicacid, maleic anhydride, fumaric acid, itaconic acid, cinnamic acid,sorbic acid, 2-(meth) acryloylethane sulfonic acid, 2-(meth)acryloylpropane sulfonic acid, 2-(meth) acrylamido-2-methylpropanesulfonic acid, vinyl sulfonic acid, styrene sulfonic acid, allylsulfonic acid, vinyl phosphonic acid and 2-(meth)acryloyloxyethylphosphate, and alkaline metal salts and alkaline earth metal salts,ammonium salts, and alkyl amine salts thereof;

dialkyl amino alkyl(meth)acrylates, such asN,N-dimethylaminoethyl(meth)acrylate and N,N-dimethylaminopropyl (meth)acrylate, and quaternary compounds thereof (for example, a reactionproduct produced with alkylhalide, and a reaction product produced withdialkyl sulfuric acid);

dialkyl amino hydroxyalkyl(meth)acrylates, and quaternary compoundsthereof;

N-alkyl vinyl pyridine halide;

hydroxyalkyl(meth)acrylates, such as hydroxymethyl (meth)acrylate,2-hydroxyethyl (meth)acrylate, and 2-hydroxypropyl (meth)acrylate;

acrylamide, methacrylamide, N-ethyl (meth) acrylamide,N-n-propyl(meth)acrylamide, N-isopropyl(meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl(meth)acrylate, methoxypolyethylene glycol (meth) acrylate, polyethyleneglycol mono(meth)acrylate, vinylpyridine, N-vinylpyrrolidone, N-acryloylpiperidine, and N-acryloyl pyrrolidine;

vinyl acetate; and

alkyl (meth)acrylates, such as methyl (meth)acrylate, and ethyl(meth)acrylate. These monomers may be used individually, or incombination.

Among the above-exemplified monomers, unsaturated monomers containing anacrylate monomer as a chief constituent are preferred because theresulting water-absorbent resins have significantly improved waterabsorption characteristics. Here, the acrylate monomers means acrylicacids and/or water-soluble salts of acrylic acids. The water-solublesalts of acrylic acids are alkaline metal salts, alkaline earth metalsalts, ammonium salts, hydroxy ammonium salts, amine salts and alkylamine salts of acrylic acids having a neutralization rate within a rangeof from 30 mole percent to 100 mole percent, more preferably within arange of from 50 mole percent to 99 mole percent. Among the exemplifiedwater-soluble salts, sodium salt and potassium salt are more preferred.These acrylate monomers may be used individually or in combination.

When the unsaturated monomer contains an acrylate monomer as a chiefconstituent, the amount of monomers other than the acrylate monomer ispreferably less than 40 weight percent, more preferably less than 30weight percent, and most preferably less than 10 weight percent of thetotal unsaturated monomer. By using the monomers other than the acrylatemonomer in the above mentioned ratio, the water absorptioncharacteristics of the resulting water-absorbent resin are furtherimproved, and the water-absorbent resin can be obtained at furtherreduced costs.

As a cross-linking agent used for polymerizing the unsaturated monomerin the present invention, for example, the following compounds arelisted. Compounds having a plurality of vinyl groups in a molecule.Compounds having at least one vinyl group in a molecule and at least onefunctional group reactive with a carboxyl group in the unsaturatedmonomer. Compounds having in a molecule a plurality of functional groupsreactive with the carboxyl group. These cross-linking agents may be usedindividually, or in combination.

Examples of the compounds having a plurality of vinyl groups in amolecule are N,N′-methylene bis(meth)acrylamide, (poly)ethylene glycoldi(meth)acrylate, (poly)propylene glycol di(meth)acrylate,trimethylolpropane tri(meth)acrylate, trimethylolpropanedi(meth)acrylate, glycerine tri(meth)acrylate, glycerine acrylatemethacrylate, ethyleneoxide denaturated trimethylolpropanetri(meth)acrylate, pentaerythritol tetra(meth)acrylate,dipentaerythritol hexa(meth)acrylate, N,N-diallyl acrylamide, triallylcyanurate, triallyl isocyanurate, triallyl phosphate, triallylamine,diallyloxy acetate, N-methyl-N-vinyl acrylamide, bis(N-vinyl carboxylicamide), and poly(meth)aliloxy alkanes such astetraallyloxy ethane.

As the compound having at least one vinyl group in a molecule and atleast one functional group reactive with the carboxylic group, forexample, ethylene unsaturated compounds having at least one hydroxylgroup, epoxy group or cationic group can be used. Example of suchcompounds are glycidyl (meth)acrylate, N-methylol acrylamide, anddimethylaminoethyl(meth)acrylate.

As the compound having a plurality of functional groups reactive withthe carboxyl group in a molecule, for example, compounds having at leasttwo hydroxyl groups, epoxy groups, cationic groups or isocyanate groupscan be used. Example of such compounds are (poly)ethylene glycoldiglycidyl ether, glycerol diglycidyl ether, ethylene glycol,polyethylene glycol, propylene glycol, glycerin, pentaerythritol,ethylenediamine, ethylene carbonate, polyethylene imine, and ammoniumsulfate.

Among the exemplified cross-linking agents, preferred compounds arewater-soluble compounds having a plurality of vinyl groups in amolecule, such as N,N′-methylene bis(meth)acrylamide, (poly)ethyleneglycol di (meth) acrylate, (poly) propylene glycol di (meth) acrylate,trimethylolpropane tri (meth) acrylate, trimethylolpropane di (meth)acrylate, glycerine tri (meth) acrylate, glycerine acrylatemethacrylate, ethyleneoxide denaturated trimethylolpropanetri(meth)acrylate, pentaerythritol tetra(meth)acrylate,dipentaerythritol hexa(meth)acrylate, triallyl cyanurate, triallylisodyanurate, triallyl phosphate, triallylamine, and poly(meth)alyloxyalkane.

The amount of the cross-linking agent with respect to the unsaturatedmonomer varies depending on a combination of unsaturated monomer andcross-linking agent. However, the cross-linking agent is used in anamount ranging preferably from 0.0001 weight parts to 10 weight parts,more preferably from 0.001 weight parts to 5 weight parts, mostpreferably from 0.01 weight parts to 2 weight parts, based on 100 partsby weight of the unsaturated monomer. When the amount of cross-linkingagent exceeds 10 weight parts, such unfavorable results are shown thatthe absorbent capacity of the resulting water-absorbent resin islowered, and foaming by a blowing agent, to be described later, becomesinsufficient. On the other hand, when the amount of cross-linking agentis less than 0.0001 weight parts, such unfavorable results are exhibitedthat the absorption rate and the gel strength of the resultingwater-absorbent resin are lowered, the water soluble component contentincreases, and the control of foaming by a blowing agent is difficult.If the unsaturated monomer is polymerized without using a blowing agent,the water absorption characteristics of the resulting water-absorbentresin and various properties of the water-absorbent resin afterabsorption become unsatisfactory.

When polymerizing the unsaturated monomer under the presence of across-linking agent, it is preferred to use an aqueous solution as theunsaturated monomer and the cross-linking agent in order to improve thewater absorption characteristics of the resulting water-absorbent resinand to achieve efficient foaming by a blowing agent. Namely, water ispreferably used as a solvent. The concentration of the unsaturatedmonomer in the aqueous solution (hereinafter referred to as the aqueousmonomer solution) is within a range of preferably from 20 weight percentto 65 weight percent, more preferably from 25 weight percent to 60weight percent, most preferably from 30 weight percent to 45 weightpercent. If the concentration of the unsaturated monomer is less than 20weight percent, the water-soluble component content in the resultingwater-absorbent resin may increase, and the absorption rate may not beimproved because foaming by the blowing agent is insufficient. On theother hand, if the concentration of the unsaturated monomer exceeds 65weight percent, it may be difficult to control the reaction temperatureand the foaming by the blowing agent.

It is also possible to use water and an organic solvent soluble in watertogether as a solvent for the aqueous monomer solution. Examples of theorganic solvent are methyl alcohol, ethyl alcohol, acetone, dimethylsulfoxide, ethylene glycol monomethyl ether, glycerin., (poly)ethyleneglycol, (poly)propylene glycol, and alkylene carbonate. These organicsolvents may be used individually, or in combination.

In this case, the amount of the organic solvent is preferably controlledso that the average particle diameter of the blowing agent dispersed iswithin a range of from 1 μm to 100 μm. More specifically, the amount ofthe organic solvent is preferably not higher than 40 percent by weightof water, more preferably not higher than 20 weight percent, mostpreferably not higher than 10 weight percent.

The blowing agent used when polymerizing the unsaturated monomer in thepresent invention is in particle form, and is a compound which isinsoluble or slightly soluble in water and in the organic solvent and issolid at normal temperatures. Example of such a blowing agent are:

organic compounds, such as azodicarbonamide, azobisisobutyronitrile,barium azodicarboxylate, dinitrosopentamethylenetetramine,4,4′-oxybis(benzen sulfonyl hydrazide), p-toluenesulfonyl hydrazide,diazoaminobenzene, N,N′-dimethyl-N,N′-dinitrosoterephthalamide,nitrourea, acetone-p-toluenesulfonyl hydrazone, p-toluenesulfonyl azide,2,4-toluenedisulfonyl hydrazide, p-methylurethane benzene sulfonylhydrazide, trinitroso trimethylene triamine, p-toluenesulfonylsemicarbazide, oxalyl hydrazide, nitroguanidine, hydroazocarbonamide,trihydrazino triamine, azobis formamide, benzenesulfonyl hydrazide,benzene-1,3-disulfonyl hydrazide, diphenyl sulfone-3,3′-disulfonylhydrazide, 4,4′-oxybis(benezene sulfonyl hydrazide), sulfone hydrazide,malonic acid and salts thereof, and carbamic acid and salts thereof;

acrylic acid salts of azo-compounds containing an amino group,represented by general formula (1)

(wherein X₁ and X₂ independently represent an alkylene group having 1 to4 carbons, R₁, R₂, R₃, R₄, R₅, and R₆ independently represent a hydrogenatom, alkyl group having 1 to 4 carbons, aryl group, allyl group orbenzyl group), or general formula (2)

(wherein X₃ and X₄ independently represent an alkylene group having 1 to4 carbons, X₅ and X₆ independently represent an alkylene group having 2to 4 carbons, and R₇ and R₈ independently represent a hydrogen atom oralkyl group having 1 to 4 carbons); and

inorganic compounds, such as carbonates including sodium bicarbonate,ammonium carbonate, ammonium bicarbonate, ammonium nitrite, basicmagnesium carbonate, and calcium carbonate. These blowing agents may beused individually, or in combination. Among the exemplified blowingagents, acrylic acid salts of azo-compound containing an amino group areparticularly preferred. The acrylic acid salt of the azo-compoundcontaining an amino group can be evenly dispersed in the aqueous monomersolution in a still state while retaining a predetermined averageparticle diameter without using a dispersing agent such as a surfaceactive agent and water-soluble polymer, and does not causesedimentation, floatation nor separation. Moreover, the acrylic acidsalts of azo-compounds containing an amino group exhibit excellentdispersion properties with respect to acrylate monomers.

The acrylic acid salts of azo-compounds containing an amino groupsrepresented by general formula (1) or (2) include, but are notnecessarily limited to, 2,2′-azobis(2-methyl-N-phenylpropionamidine)diacrylate, 2,2′-azobis[N-(4-chlorophenyl)-2-methyl propion amidine]diacrylate, 2,2′-azobis[N-(4-hydroxyphenyl)-2-methyl propion amidine]diacrylate, 2,2′-azobis[2-methyl-N-(phenylmethyl)-propionamidine]diacrylate, 2,2′-azobis[2-methyl-N-(2-propenyl)-propionamidine]diacrylate, 2,2′-azobis(2-methyl propion amidine) diacrylate,2,2′-azobis[N-(2-hydroxyethyl)-2-methylpropionamidine] diacrylate,2,2′-azobis [2-(5-methyl-2-imidazolin-2-yl)propane] diacrylate,2,2′-azobis[2-(2-imidazolin-2-yl)propane] diacrylate,2,2′-azobis-[2-(4,5,6,7-tetrahydro-1H-1,3-diacepine-2-yl)propane]diacrylate,2,2′-azobis[2-(3,4,5,6-tetrahydropyrimidine-2-yl)propane]diacrylate,2,2′-azobis[2-(5-hydroxy-3,4,5,6-tetrahydropyrimidine-2-yl)propane]diacrylate, and2,2′-azobis{2-[1-(2-hydroxyethyl)-2-imidazoline-2-yl]propane}diacrylate. Among the above-exemplified acrylic acid salts ofazo-compounds containing an amino group, 2,2′-azobis(2-methyl propionamidine) diacrylate is particularly preferred.

The acrylic acid salt of azo-compounds containing an amino group can beeasily isolated by precipitating the acrylic acid salts in, for example,an aqueous monomer solution and then filtering. When precipitating theacrylic acid salts of azo-compound containing an amino group in anaqueous monomer solution, a poor solvent may be added or cooling may beperformed, if necessary.

As the blowing agent, a blowing agent prepared beforehand may be addedto the aqueous monomer solution, or a blowing agent may be prepared bydissolving a precursor of the blowing agent (hereinafter referred to asthe blowing agent precursor) in the aqueous monomer solution, and thenadding carbon dioxide gas and acrylic acid salt to the aqueous monomersolution, if necessary. Namely, it is possible to precipitate theblowing agent by reacting the blowing agent precursor with the carbondioxide gas and acrylic acid salt in the aqueous monomer solution. Apreferred acrylic acid salt is sodium acrylate. When the unsaturatedmonomer is an acrylate monomer, the unsaturated monomer can function asthe acrylic acid salt.

The acrylic acid salts of azo-compounds containing an amino groupfunction as both the blowing agent and the radical polymerizationinitiator. By polymerizing the unsaturated monomer under the presence ofthe acrylic acid salt of the azo-compound containing an amino group, itis possible to obtain a water-absorbent resin having further reducedwater-soluble component content and residual monomer content. Morespecifically, by using the acrylic acid salt of the azo-compoundcontaining an amino group, it is possible to obtain a water-absorbentresin containing the water-soluble component in an amount of not higherthan 15 weight percent, preferably within a range of from 1 weightpercent to 10 weight percent, and the residual monomers in an amount ofnot higher than 500 ppm, preferably not higher than 300 ppm, morepreferably not higher than 100 ppm.

The amount of the blowing agent with respect to the unsaturated monomeris not particularly limited, and is suitably decided depending on acombination of the unsaturated monomer and the blowing agent. However,the blowing agent is used in an amount ranging preferably from 0.005weight parts to 25 weight parts, more preferably from 0.01 weight partsto 5 weight parts, most preferably from 0.05 weight parts to 2.5 weightparts, based on 100 parts by weight of the unsaturated monomer. When theamount of the blowing agent is out of the above-mentioned ranges, theresulting water-absorbent resin may not have sufficient water absorptioncharacteristics.

The average particle diameter of the blowing agent which is present in adispersed state during polymerization is within a range of preferablyfrom 1 μm to 100 μm, more preferably from 2 μm to 50 μm, most preferablyfrom 3 μm to 40 μm. By setting the average particle diameter of theblowing agent within the above-mentioned range, it is possible of adjustthe average pore diameter of the water-absorbent resin within a range offrom 10 μm to 500 μm, more preferably from 20 μm to 400 μm, still morepreferably from 30 μm to 300 μm, most preferably from 50 μm to 200 μm,thereby improving the water absorption characteristics of thewater-absorbent resin (for example, the dispersion and absorption rateof the aqueous fluid). Namely, it is possible to set the average porediameter of the water-absorbent resin within a desired range by settingthe average particle diameter of the blowing agent.

If the average particle diameter of the blowing agent is smaller than 1μm or if the blowing agent is dissolved in the aqueous monomer solution,the degree of foaming becomes insufficient, and the average porediameter of the water-absorbent resin cannot be adjusted within thedesired range. On the other hand, if the average particle diameter ofthe blowing agent is larger than 100 μm, the average pore diameter ofthe water-absorbent resin cannot be adjusted within the desired range.In addition, a decrease in the gel strength of the water-absorbent resinand an increase in the water-soluble component content unfavorablyoccur. The average particle diameter of the blowing agent in the aqueousmonomer solution can be easily measured with a laser-type particle sizedistribution apparatus.

As the blowing agent precursor when the blowing agent is an inorganiccompound, for example, calcium hydroxide and magnesium hydroxide aregiven.

When the blowing agent is acrylic acid salt of the azo-compoundcontaining an amino group, the blowing agent precursor is hydrochlorideof the azo-compound containing an amino group. Examples include2,2′-azobis(2-methyl-N-phenyl propion amidine) dihydrochloride,2,2′-azobis [N-(4-chlorophenyl)-2-methylpropion amidine]dihydrochloride, 2,2′-azobis[N-(4-hydroxyphenyl)-2-methylpropionamidine] dihydrochloride, 2,2′-azobis[2-methyl-N-(phenylmethyl)-propionamidine] dihydrochloride, 2,2′-azobis[2-methyl-N-(2-propenyl)-propionamidine] dihydrochloride, 2,2′-azobis(2-methyl propion amidine)dihydrochloride, 2,2′-azobis[N-(2-hydroxyethyl)-2-methylpropion amidine]dihydrochloride, 2,2′-azobis[2-(5-methyl-2-imidazoline-2-yl)propane]dihydrochloride, 2,2′-azobis[2-(2-imidazoline-2-yl)propane]dihydrochloride,2,2′-azobis[2-(4,5,6,7-tetrahydro-1H-1,3-diacepine-2-yl)propane]dihydrochloride,2,2′-azobis [2-(3,4,5,6-tetrahydropyrimidine-2-yl)propane]dihydrochloride,2,2′-azobis[2-(5-hydroxy-3,4,5,6-tetrahydropyrimidine-2-yl)propane]dihydrochloride, and2,2′-azobis{2-[1-(2-hydroxyethyl)-2-imidazoline-2-yl]propane}dihydrochloride. These dihydrochlorides of azo-compounds containing anamino group are heat decomposable azo polymerization initiators.

The hydrochloride of azo-compound containing an amino group causessedimentation, floatation and separation if the solubility in theaqueous monomer solution is low. Therefore, if the hydrochloride ofazo-compound containing an amino group is directly used as a blowingagent, a water-absorbent resin having excellent water absorptioncharacteristics cannot be obtained.

The condition for producing the acrylic acid salt of azo-compoundcontaining an amino group by reacting the hydrochloride of theazo-compound containing an amino group with an acrylic acid salt is notparticularly limited, but the following conditions are preferred. It ispreferred to arrange the pore diameter of the resulting water-absorbentresin to a desired size by freely setting the conditions and suitablyadjusting the particle diameter during dispersion of the acrylic acidsalt of the azo-compound containing an amino group.

Specifically, the set temperature is within a range of preferably from−10° C. to 50° C., more preferably from 0° C. to 40° C. As the acrylicacid salts, acrylic alkaline metal salts are preferred, and sodiumacrylate is more preferred. The neutralization rate of the acrylic acidsalt is preferably not lower than 50 mole percent, more preferably notlower than 70 mole percent. The concentration of the acrylic acid saltin the aqueous monomer solution is within a range of preferably from 20weight percent to a saturated concentration, more preferably from 25weight percent to the saturation concentration.

Moreover, when producing the acrylic acid salts of azo-compoundscontaining an amino group, it is desirable to agitate the aqueousmonomer solution. By agitating the aqueous monomer solution at a ratenot lower than 10 rpm, more preferably at rates ranging from 20 rpm to10,000 rpm, it is possible to prepare the acrylic acid salt ofazo-compound containing an amino group having a substantially uniformparticle diameter within a short time. The prepared acrylic acid salt ofazo-compound containing an amino group can be directly used for thepolymerization of an unsaturated monomer without the necessity ofisolation.

As a method of producing acrylic acid salts of azo-compounds containingan amino group in an aqueous monomer solution, i.e., a method ofdispersing acrylic acid salts of azo-compounds containing an amino groupin an aqueous monomer solution, for example, the following two methodsare given. A method of producing an aqueous monomer solution by addingthe hydrochloride of azo-compound containing an amino group to anacrylic acid salt having a neutralization rate of 100 percent to preparean acrylic acid salt of the azo-compound containing an amino group, andthen mixing an unsaturated monomer, such as an acrylic acid which hasnot been neutralized, a cross-linking agent, and a solvent if necessary,with the acrylic acid salt. A method of producing an aqueous monomersolution in which an acrylic acid salt of azo-compound containing anamino group is dispersed by adding hydrochloride of the azo-compoundcontaining an amino group, and an acrylic acid salt if necessary, to anaqueous monomer solution which is prepared beforehand. The latter methodis more preferred because it can more efficiently produce the acrylicacid salt of the azo-compound containing an amino group with a uniform,particle diameter. It is also possible to adjust the concentration ofthe unsaturated monomer in the aqueous monomer solution to a desiredlevel by adding a solvent such as water to the aqueous monomer solutionafter producing the acrylic acid salt of the azo-compound containing anamino group.

The following description will discuss a process for producing awater-absorbent resin according to the present invention.

The water-absorbent resin of the present invention is obtained bydispersing a blowing agent in an aqueous monomer solution and thenpolymerizing an unsaturated monomer (aqueous solution polymerization).

The method of dispersing the blowing agent in the aqueous monomersolution is not particularly limited. Examples include: a method ofdispersing a blowing agent by adding the blowing agent to an aqueousmonomer solution; a method of dispersing a blowing agent by adding ablowing agent precursor to an aqueous monomer solution and thenproducing and dispersing the blowing agent in the aqueous monomersolution; and a method of dispersing a blowing agent by producing anaqueous monomer solution by adding an unsaturated monomer, across-linking agent and a blowing agent to a solvent such as water anddispersing the blowing agent. Among these methods, a preferred method isthe method of dispersing a blowing agent by adding a blowing agentprecursor to an aqueous monomer solution and then producing anddispersing the blowing agent in the aqueous monomer solution becausethis method produces a water-absorbent resin having a further improvedwater absorption characteristics. When dispersing the blowing agent, theaqueous monomer solution may be agitated, or may not be agitated.

However, when the blowing agent is an inorganic compound such ascarbonate salt and the unsaturated monomer includes an acrylate monomeras a chief constituent, since the reactivity of the inorganic compoundand the acrylate monomer is relatively high, it is difficult to dispersethe inorganic compound in the aqueous monomer solution and control theparticle diameter thereof. In this case, it is; desirable to dispersethe inorganic compound in the aqueous monomer solution by using adispersion stabilizer such as a surface-active agent and water-solublepolymer.

When the blowing agent is carbonate salt, preferred examples of thedispersion stabilizer are:

hydrophilic organic solvents, such as methyl alcohol, ethyl alcohol,propyl alcohol, butyl alcohol, acetonitrile, and dimethyl formamide;

water-soluble polymers, such as polyvinyl alcohol, starches and theirderivatives, polygalacto mannan, cellulose including methyl cellulose,carboxymethyl cellulose and hydroxyethyl cellulose and theirderivatives, polyalkylene oxides, polyacrylic acids, and polyacrylicacid salts;

anionic surface active agents, such as fatty acid salts of sodium oleateand potassium caster oil, alkylsulfuric ester salts of lauryl sodiumsulfide and lauryl ammonium sulfide, alkylbenzene sulfonic acid saltsincluding dodecyl benzene sodium sulfonic acid salt, alkyl naphthalenesulfonic acid salt, dialkyl sulfo-succinate, alkyl phosphate salt, andnaphthalenesulfonic formalized condensation product, and polyoxyethylenealkyl sulfate salt;

nonionic surface active agents, such as polyoxyethylene alkyl ether,polyoxyethylene alkyl phenol ether, polyoxyethylene fatty acid ester,sorbitan fatty acid ester, polyoxy sorbitan fatty acid ester,polyoxyethylene alkylamine, fatty acid esters, andoxyethylene-oxypropylene block polymer;

cationic surface active agents, such as alkyl amine salts includinglauryl amine acetate and stearyl amine acetate, quaternary ammoniumsalts including lauryl trimethyl ammonium chloride and stearyl trimethylammonium chloride; and

amphoteric ionic surface active agents, such as lauryl dimethylamineoxide. However, it is not necessary to restrict the dispersionstabilizer to those mentioned above. Such dispersion stabilizers may beused individually, or in combination.

Among the above-exemplified dispersion stabilizers, it is preferred touse at least one kind of dispersion stabilizer selected from the groupconsisting of water-soluble polymer and surface active agent. It is morepreferred to use both the water-soluble polymer and surface activeagent. Among the water-soluble polymers, polyvinyl alcohol, starches andtheir derivatives, cellulose and the derivatives are preferred. Thepolyvinyl alcohol and hydroxyethyl cellulose are particularly preferred.Partially saponified polyvinyl alcohol is still more preferred. Amongthe surface active agents, anionic surface active agents and nonionicsurface active agents are preferred. Nonionic surface active agentshaving an HLB of not lower than 7 are particularly preferred.

By adding such a dispersion stabilizer to the aqueous monomer solution,it is possible to evenly disperse an inorganic compound (blowing agent)such as carbonate in the aqueous monomer solution, and control theaverage particle diameter of the inorganic compound within a range offrom 1 μm to 100 μm. The amount of the dispersion stabilizer withrespect to the blowing agent is suitably set according to thecombination of the blowing agent and the dispersion stabilizer. Theamount of the dispersion stabilizer to be used is not necessarilylimited, but is preferably no more than 5 weight parts based on 100parts by weight of unsaturated monomer, and preferably no more than 500weight parts, more preferably no more than 100 weight parts, still morepreferably no more than 50 weight parts, most preferably no more than 10weight parts based on 100 parts by weight of the blowing agent. Morespecifically, the amount of the dispersion stabilizer to be used iswithin a range preferably from 0.01 weight parts to 500 weight parts,more preferably from 0.05 weight parts to 100 weight parts, still morepreferably from 0.5 weight parts to 50 weight parts, most preferablyfrom 0.5 weight parts to 10 weight parts.

The unsaturated monomer in the aqueous monomer solution in which theblowing agent is dispersed can be polymerized by a known method. Thepolymerization method is not particularly limited, and various methodscan be used. Examples include radical polymerization using a, radicalpolymerization initiator, irradiation-induced polymerization, electronradiation-induced polymerization, and ultraviolet-induced polymerizationusing a photosensitizer. Among these methods, radical polymerization ismore preferred because this method can quantitatively and perfectlypolymerize the unsaturated monomer.

As the radical polymerization, there are various Polymerization methods,such as aqueous solution polymerization, cast polymerization which isperformed within a mold, thin-layer polymerization which is performed ona belt conveyer, polymerization which is performed while makinggenerated hydrogel polymer into small pieces, reversed-phase suspensionpolymerization, reversed-phase emulsion polymerization, precipitationpolymerization, and bulk polymerization. Among these polymerizationmethods, the aqueous solution polymerization which polymerizes theunsaturated monomer in the form of aqueous solution is more preferredbecause the polymerization temperature can be easily controlled.

The aqueous solution polymerization of the unsaturated monomer may beperformed either continuously or batch-wise, or may be performed undersuction, pressure, or atmospheric pressure. The polymerization ispreferably performed in the flow of inactive gas, such as nitrogen,helium, argon, or carbonate gas.

When performing the aqueous solution polymerization, it is preferred todissolve or disperse a radical polymerization initiator in an aqueousmonomer solution in advance. Examples of the radical polymerizationinitiator include:

peroxides, such as ammonium persulfate, potassium persulfate, sodiumpersulfate, hydrogen peroxide, benzoyl peroxide, cumene hydroperoxide,and di-t-butyl peroxide;

redox initiators formed by combining the above-mentioned peroxides andreducing agents, such as sulfite, bisulfite, thiosulfate, formamidinesulfinic acid, and. ascorbic acid;

acrylic acid salts of azo-compound containing an amino group representedby general formula (1) or (2) above; and

azo polymerization initiators, such as hydrochlorides of theazo-compound containing an amino group. These radical polymerizationinitiators may be used individually, or in combination. When the acrylicacid salt of azo-compound containing an amino group is used as theradical. polymerization imitator, it is more preferred to use a, redoxinitiator together with the acrylic acid salt.

The amount of the radical polymerization initiator with respect to theunsaturated monomer is varied depending on the combination of theunsaturated monomer and the radical polymerization initiator. However,the amount of the radical polymerization initiator to be used is withina range of preferably from 0.0005 weight parts to 5 weight parts, morepreferably from 0.005 weight parts to 2.5 weight parts, based on 100parts by weight of the unsaturated monomer. If the amount of the radicalpolymerization initiator is less than 0.0005 weight parts, the amount ofunreacted unsaturated monomers increases, causing an unfavorableincrease of the residual monomer content in the resultingwater-absorbent resin. On the other hand, if the amount of the radicalpolymerization initiator exceeds 5 weight parts, an unfavorable increaseof the water-soluble component content in the resulting water-absorbentresin occurs.

Although the temperature at the initiation of polymerization variesdepending on the type of a radical. polymerization initiator used, it ispreferably within a, range of from 0° C. to 40° C., more preferably from10° C. to 30° C. Similarly, although the polymerization temperatureduring the reaction varies depending on the type of a radicalpolymerization initiator used, it is preferably within a range of from40° C. to 120° C., more preferably from 50° C. to 110° C. If thetemperature at the initiation of polymerization or the polymerizationtemperature during the reaction is outside of the above-mentioned range,unfavorable results may be exhibited, for example, the residual monomercontent in the resulting water-absorbent resin increases, the control offoaming by a blowing agent becomes difficult, and the absorbent capacityof the water-absorbent resin is lowered because of an excessiveself-cross-linking reaction.

The reaction time is not necessarily limited, but is preferably setaccording to the combination of the unsaturated monomer, cross-linkingagent and radical polymerization initiator, or reaction conditions suchas the reaction temperature. Moreover, the time between the dispersionof the blowing agent and the initiation of the polymerization of theunsaturated monomer is not necessarily limited, but a relatively shorttime is preferred.

When performing the aqueous solution polymerization, the aqueous monomersolution may be agitated or may not be agitated. However, in order toachieve efficient foaming by the blowing agent, it is desirable to keepthe aqueous monomer solution at rest for at least a predetermined periodof time during the reaction. Foaming by the blowing agent is moreefficiently performed by keeping the aqueous monomer solution at restafter the initiation of polymerization until the polymerization ratereaches 10 percent, more preferably 30 percent, still more preferably 50percent, most preferably until the end of polymerization. When using theacrylic acid salt of an azo-compound containing an amino grouprepresented by general formula (1) or (2) above as the blowing agent,the polymerization may be carried out while performing agitation fromthe initiation of polymerization until the end of polymerization, i.e.,the entire polymerization is performed under the agitated condition.

A hydrogel containing cells which is a (co)polymer of the unsaturatedmonomer is produced by the polymerization. More specifically, thehydrogel containing cells is produced as follows. The unsaturatedmonomer is (co)polymerized, a cross-linking reaction by thecross-linking agent and foaming by the blowing agent proceed, and holes(voids) are formed in the (co)polymer.

The hydrogel containing cells is chopped into pieces of a size rangingfrom about 0.1 mm to about 50 mm by a predetermined method during orafter the reaction depending on the need. Subsequently, in order toachieve more efficient foaming, the hydrogel containing cells is dried.It is possible to perform foaming by the blowing agent during dryinginstead of during reaction.

The drying temperature is not particularly limited., but is within arange of, for example, preferably from 100° C. to 300° C., morepreferably from 120° C. to 220° C. in order to perform more efficientfoaming. Similarly, the drying time is not particularly limited, but ispreferably between 10 seconds and 3 hours. The hydrogel containing cellsmay be neutralized or may be chopped into smaller pieces prior todrying.

The drying method is not particularly limited. Drying may be performedby various methods, for example, heat drying, hot-air drying, vacuumdrying, infrared drying, microwave drying, drum dryer drying,dehydrating by forming an azeotrope with a hydrophobic organic solvent,and high-humidity drying using high-temperature water vapor. Among thesedrying methods, hot-air drying and microwave drying are more preferred.In particular, the microwave drying is preferred. When the hydrogelcontaining cells is irradiated with a microwave, the cells swell intosize several times or several tens times larger than the size beforeirradiated, thereby producing a water-absorbent resin having furtherimproved absorption rate.

When microwave-drying the hydrogel containing cells, it is desirable toarrange the chopped hydrogel containing cells to have a thickness ofpreferably not less than 3 mm, more preferably not less than 5 mm, mostpreferably not less than 10 mm. In addition, when microwave-drying thehydrogel containing cells, it is particularly desirable to shape thehydrogel containing cells into a sheet having the above-mentionedthickness.

The water-absorbent resin of the present invention can be easilyobtained at inexpensive costs by the above-mentioned polymerization,i.e., the above-mentioned production process. The water-absorbent resinis a porous cross-linking polymer having holes formed uniformlythroughout the water-absorbent resin. The molar weight is relativelylarge, and the average pore diameter is within a range of preferablyfrom 10 μm to 500 μm, more preferably from 20 μm to 400 μm, still morepreferably from 30 μm to 300 μm, most preferably from 50 μm to 200 μm.The sheet-like water-absorbent resin obtained by microwave-drying thehydrogel containing cells has a bulk specific gravity within a range offrom 0.01 g/cm³to 0.5 g/cm³.

The average pore diameter is obtained by an image analysis of theprofile of the dried water-absorbent resin with an electron microscope.More specifically, a histogram indicating the distribution of porediameters of the water-absorbent resin is prepared by performing theimage analysis, and the average pore diameter is obtained by calculatingthe number average of the pore diameters from the histogram.

Since the water-absorbent resin is porous having the above-mentionedaverage pore diameter, a sufficient liquid guide space which isnecessary for an aqueous fluid to move into the water-absorbent resin isensured under no pressure and under pressure conditions. Thus, thewater-absorbent resin achieves excellent permeability and improveddispersion of the aqueous fluid, and increases the absorption rate andwater retention capacity by capillarity. Moreover, since thewater-absorbent resin is porous, even if the water-absorbent resin is inthe form of particles, it is possible to retain the liquid permeabilitybetween the particles, thereby preventing a so-called gel blockingphenomenon. An average pore diameter smaller than 10 μm is not preferredbecause the liquid permeability and dispersibility in the aqueoussolution is declined. Al average pore diameter larger than 500 μm is notpreferred because the absorption rate is lowered.

The form and size (particle diameter) of the water-absorbent resin isnot particularly limited, and can be suitably set according to the useof the water-absorbent resin. For example, the water-absorbent resin maybe shaped into various forms, such as a sheet and a block. However, whenusing the water-absorbent resin as a sanitary material, it is desirableto perform grinding and classifying processes so as to arrange theaverage particle diameter within a range, preferably between 50 μm and1,000 μm, more preferably between 150 μm and 800 μm, most preferablybetween 200 μm and 600 μm. It is also possible to form thewater-absorbent resin into particles by granulation.

The water-absorbent resin with the above-mentioned structure may betreated with a surface cross-linking agent to form a covalent bond(secondary cross-linkages) and further increase the cross-link densityin the vicinity of the surface. The surface cross-linking agent is notparticularly limited as long as it is a compound having a plurality offunctional groups capable of reacting with a carboxyl group of thewater-absorbent resin to form a covalent band. By treating thewater-absorbent resin with the surface cross-linking agent, the liquidpermeability, the absorption rate of the water-absorbent resin, and theabsorbent capacity and liquid permeability under pressure (to bedescribed later) are further improved.

The surface cross-linking agent is not particularly limited, andpreferred examples include:

polyhydric alcohol compounds, such as ethylene glycol, diethyleneglycol, triethylene glycol, tetraethylene glycol, polyethylene glycol,propylene glycol, 1,3-propanediol, dipropylene glycol,2,2,4-trimethyl-1,3-pentanediol, polypropylene glycol, glycerin,polyglycerin, 2-butene-1,4-diol, 1,4-butanediol, 1,5-pentanediol,1,6-hexanediol, 1,2-cyclohexanedimethanol, 1,2-cyclohexanediol,trimethylolpropane, diethanolamine, triethanolamine, polyoxypropylene,oxyethylene-oxypropylene block copolymer, pentaerythritol, and sorbitol;

epoxy compounds, such as ethylene glycol diglycidyl ether, polyethyleneglycol diglycidyl ether, glycerol polyglycidyl ether, diglycerolpolyglycidyl ether, polyglycerol polyglycidyl ether, propylene glycoldiglycidyl ether, polypropylene glycol diglycidyl ether, and glycidol;

polyamine compounds, such as ethylenediamine, diethylenetriamine,triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine,polyethylene-imine, and polyamide-polyamine;

haloepoxy compounds, such as epichlorohydrin, epibromohydrin, andα-methylepichlorohydrin;

condensation products of the above-mentioned polyamine compounds andhaloepoxy compounds;

polyisocyanate compounds, such as 2,4-trilene diisocyanate, andhexamethylene diisocyanate;

polyoxazoline compounds, such as 1,2-ethylenebisoxazoline;

silane coupling agents, such as γ-glycidoxypropyltrimetoxysilane, andγ-aminopropyltrimetoxysilane; and

alkylene carbonate compounds, such as 1,3-dioxolan-2-one,4-methyl-1,3-dioxolan-2-one, 4,5-dimethyl-1,3-dioxolan-2-one,4,4-dimethyl-1,3-dioxolan-2-one, 4-ethyl-1,3-dioxolan-2-one,4-hydroxymethyl-1,3-dioxolan-2-one, 1,3-dioxane-2-one,4-methyl-1,3-dioxane-2-one, 4,6-. dimethyl-1,3-dioxane-2-one, and1,3-dioxopan-2-one. Among the exemplified surface cross-linking agents,the polyhydric alcohol compounds, epoxy compounds, polyamine compounds,condensation products of the polyamine compounds and haloepoxycompounds, and alkylene carbonate compounds are more preferred.

These surface cross-linking agents may be used individually, or incombination. When using more than ones type of surface cross-linkingagents, it is possible to obtain a water-absorbent resin having furtherimproved absorbent characteristics by using a combination of a firstsurface cross-linking agent and a second surface cross-linking agenthaving different solubility parameters (SP values). The solubilityparameter is a value which is generally used as a factor representingthe polarity of a compound.

The first surface cross-linking agent is a compound which is reactivewith the carboxyl group of the water-absorbent resin and has asolubility parameter of not lower than 12.5 (cal/cm³)^(½), for example,glycerin. The second surface cross-linking agent is a compound which isreactive with the carboxyl group of the water-absorbent resin and has asolubility parameter less than 12.5 (cal/cm³)^(½), for example, ethyleneglycol diglycidyl ether.

The amount of the surface cross-linking agent with respect to thewater-absorbent resin varies depending on the combination ofwater-absorbent resin and surface cross-linking agent. However, theamount of the surface cross-linking agent to be used is within a rangeof preferably from 0.01 weight parts to 5 weight parts, more preferablyfrom 0.05 weight parts to 3 weight parts, based on 100 parts by weightof the dry water-absorbent resin. By using the surface cross-linkingagent in an amount within the above-mentioned range, it is possible tofurther improve the water absorption characteristics with respect tobody fluids (aqueous fluids), such as urine, sweat, and blood. If theamount of the surface cross-linking agent is less than 0.01 weightparts, the cross-link density in the vicinity of the surface of thewater-absorbent resin can hardly be increased. On the other hand, if theamount of the surface cross-linking agent exceeds 5 weight parts, thesurface cross-linking agent becomes excessive, causing uneconomicalresults and difficulty in controlling the cross-link density to be asuitable value.

The method of treating the water-absorbent resin with the surfacecross-linking agent is not particularly limited. For example, thefollowing three methods are listed. {circle around (1)} A methodincluding mixing the water-absorbent resin and the surface cross-linkingagent without the presence of solvent. {circle around (2)} A methodincluding dispersing the water-absorbent resin in a hydrophobic solventsuch as cyclohexane and pentane, and mixing the surface cross-linkingagent and the water-absorbent resin. {circle around (3)} A methodincluding dispersing or dissolving the surface cross-linking agent in ahydrophilic solvent, and spraying or dropping the solution or thedispersion into the water-absorbent resin to mix them. Among thesemethods, {circle around (3)} is most preferred. Water, or a mixture ofwater and an organic solvent soluble in water is suitably used as thehydrophilic solvent.

Examples of the organic solvent include:

lower alcohols, such as methyl alcohol, ethyl alcohol, n-propyl alcohol,isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, an d t-butylalcohol;

ketones, such as acetone;

ethers, such as dioxane, ethyleneoxide (EO) added compound of monohydricalcohol, and tetrahydrofuran;

amides, such as N,N-dimethylformamide, and ε-caprolactam; and

sulfoxides, such as dimethyl sulfoxide. These organic solvents may beused individually, or in combination.

The amount of the hydrophilic solvent with respect to thewater-absorbent resin and the surface cross-linking agent variesdepending on the combination of water-absorbent resin, surfacecross-linking agent and hydrophilic solvent. However, the amount ofhydrophilic solvent to be used is preferably not higher than 200 weightparts, more preferably within a range of from 0.01 weight parts to 50weight parts, still more preferably from 0.1 weight parts to 50 weightparts, most preferably from 0.5 weight parts to 20 weight parts, basedon 100 parts by weight of the water-absorbent resin.

A mixer for use in mixing the water-absorbent resin and the surfacecross-linking agent preferably has a great mixing power so as to mixthem evenly and surely. Preferred examples of the mixer are acylindrical mixer, double-wall conical mixer, high-speed agitation-typemixer, V-shaped mixer, ribbon blender, screw mixer, fluid oven rotarydesk mixer, airborne mixer, double-arm kneader, internal mixer,crush-type kneader, rotary mixer, and screw extruder.

The treatment temperature and treatment time when treating thewater-absorbent resin with a surface cross-linking agent are notparticularly limited, and are set according to the combination ofwater-absorbent resin and surface cross-linking agent, and a desiredcross-link density. However, a preferred treatment temperature is, forexample, within a range of from 0° C. to 250° C.

When treating the water-absorbent resin with a surface cross-linkingagent, it is possible to further add a mixing assistant, if necessary.As the mixing assistant, powder of fine particles which are insoluble inwater, surface active agents, organic acids, inorganic acids, andpolyamino-acids are listed. Examples of organic acids are saturatedcarboxylic acids, such as citric acid, lactic acid, and succinic acid.As the inorganic acids, for example, phosphoric acid, sulfuric acid, andhydrochloric acid are given. These mixing assistants may be usedindividually, or in combination. The mixing assistant is preferably usedin an amount ranging from 0.01 weight parts to 5 weight parts based on100 parts by weight of the water-absorbent resin. The method for mixingthe water-absorbent resin, the surface cross-linking agent and themixing assistant is not particularly limited.

The water-absorbent resin whose cross-link density in the vicinity ofthe surface is improved by the formation of a covalent bond, i.e., thewater-absorbent resin to which the above-mentioned treatment has beenapplied, may have further improved cross-link density in the vicinity ofthe surface by treating it with a cationic compound to form an ionicbond (secondary cross linkage). The cationic compound is notparticularly limited as along as it is a compound capable of forming anionic bond by reacting with the carboxylic group of the water-absorbentresin (i.e., the carboxylic group which has not reacted with the surfacecross-linking agent). By treating the water-absorbent resin -with thecationic compound, it is possible to further improve the waterabsorption characteristics, such as the absorption rate, dispersion,water retention capacity, dry touch, and absorbent capacity underpressure.

Examples of the cationic compound are:

low-molecular-weight polyamine compounds, such as ethylenediamine,diethylenetriamine, triethylenetetramine, tetraethylenepentamine, andpentaethylenehexamine;

cationic polyelectrolytes, such as polyethylene-imine, modifiedpolyethylene-imine modified to be water-soluble by epihalohydrin,polyamine, polyamide-amine modified by grafting ethylene-imine,protonated polyamide-amine, polyetheramine, polyvinylamine,polyalkylamine, polyvinylimidazole, polyvinylpyridine,polyvinylimidazoline, polyvinyl tetrahydropyridine, polydialkylaminoalkylvinyl ether, polydialkylamino alkyl (meth) acrylate, andpolyallylamine, and salts thereof; and

polyvalent metal compounds, such as hydroxides, chlorides, sulfate, andcarbonate of polyvalent metals including zinc, calcium, magnesium,aluminum, iron and zirconium. However, the cationic compound is notnecessarily limited to those mentioned above. These cationic compoundsmay be used individually, or in combination. Among the exemplifiedcationic compounds, the cationic polyelectrolytes and salts thereof aremore preferred.

The amount of the cationic compound with respect to the water-absorbentresin varies depending on the combination of water-absorbent resin andcationic compound. However, the cationic compound is used in an amountranging preferably from 0.01 weight parts to 5 weight parts, morepreferably from 0.1 weight parts to 3 weight parts, based on 100 partsby weight of the dried water-absorbent resin. By using the cationiccompound in an amount within the above-mentioned range, it is possibleto obtain a water-absorbent resin having further improved waterabsorption characteristics, such as the absorption rate, dispersion,water retention capacity, dry touch, and absorbent capacity underpressure. The method of treating the water-absorbent resin with thecationic compound is the same as that used for treating thewater-absorbent resin with the surface cross-linking agent. Thetreatment temperature and treatment time are not particularly limited,and are set according to the combination of water-absorbent resin andcationic compound, and a desired cross-link density. However, apreferred treatment temperature is, for example, room temperature, andmay be increased to temperatures ranging from 50° C. to 100° C., ifnecessary.

By the above-mentioned method, it is possible to easily and industriallyproduce the water-absorbent resin at an inexpensive cost. The resultingwater-absorbent resin is porous having an average pore diameter within arange of from 10 μm to 500 μm. The amount of water absorbed by thewater-absorbent resin 60 minutes after the initiation of absorbentcapacity under pressure is preferably no lower than 25 g/g, morepreferably no lower than 30 g/g. The water-soluble component content inthe water-absorbent resin is preferably no higher than 15 weightpercent, more preferably within a range of from 1 weight percent to 10weight percent. Moreover, the residual monomer content in thewater-absorbent resin is preferably no higher than 500 ppm, morepreferably no higher than 300 ppm, and most preferably no higher than100 ppm. Since the physical properties of the water-absorbent resin aresuperior and balanced, the water absorption characteristics of thewater-absorbent resin, such as the liquid permeability under pressure,are excellent.

Furthermore, it is possible to impart various functions to thewater-absorbent resin by adding thereto deodorant, perfume, variousinorganic powders, blowing agent, pigment, dye, hydrophilic short fiber,plasticizer, thickening agent, surface-active agent, fertilizer,oxidizer, reducing agent, water and salts, if necessary.

A water-absorbent resin composition of the present invention is obtainedby mixing inorganic powder in the form of fine particles and theabove-mentioned water-absorbent resin, i.e., the water-absorbent resinparticles whose average particle diameter is arranged within a range ofpreferably from 50 μm to 1,000 μm, more preferably from 150 μm to 800μm, most preferably from 200 μm to 600 μm, through the gliding andclassifying steps. The water-absorbent resin composition preferablycontains a water-absorbent resin formed by an unsaturated monomercontaining an acrylate monomer as a chief constituent.

As the inorganic powder, inactive substances which are inactive withrespect to the aqueous fluid and the like, for example, fine particlesof various inorganic compounds and fine particles of clay mineral areused. Preferred inorganic compounds have suitable affinity with respectto water, and insoluble or slightly-soluble in water. Examples of suchinorganic compounds are metal oxides, such as silicon dioxide andtitanium oxide, silicic acid (salt) such as natural zeolite andsynthetic zeolite, kaolin, talc, clay, and bentonite. Silicon dioxideand silicic acid (salt) are more preferred. In particular, silicondioxide and silicic acid (salt) whose average particle diameter is notlarger than 200 μm when measured by a Coulter Counter are mostpreferred.

The amount of inorganic powder to be used varies depending on thecombination of water-absorbent resin and inorganic power. The inorganicpowder is used in an amount ranging preferably from 0.001 weight partsto 10 weight parts, more preferably from 0.01 weight parts to 5 weightparts, based on 100 parts by weight of the water-absorbent resin. Themethod of mixing the water-absorbent resin and inorganic powder is notparticularly limited. For example, a dry blending method or a wetblending is used. In particular, dry blending is preferred.

Regarding a water-absorbent resin composition of the above-mentionedstructure, the water retention capacity is not lower than 20 g/g, theabsorption rate is not higher than 120 seconds, and the liquidpermeability is not higher than 200 seconds. The water-absorbent resincomposition is made into an absorbent article by, for example, beingcomposite (combined) with fibrous material such as pulp.

The absorbent article is not particularly limited, and examplesincludes: sanitary materials (body fluids absorbent articles) such aspaper diapers, sanitary napkins, incontinence pads, wound protectingmaterial and wound healing material; absorbent articles for absorbingurine of pets; materials of construction and building, such as buildingmaterial, water retentive material for soil, packing material, and gelpusule; materials for food, such as drip absorbing material, freshnessretentive material, and heat insulating material; various industrialarticles, such as oil and water separating material, condensationpreventing material, and coagulant; and agricultural and horticulturalarticles, such as water retentive material for plant and soil. Forinstance, paper diaper is formed by layering a back sheet (backmaterial) made of waterproof material, the above-mentionedwater-absorbent resin composition, and a top sheet (front material) madeof a liquid permeable material in this order, fix these materials toeach other, and fastening a gather (an elastic section) and a so-calledtape fastener onto the laminate. The paper diaper includespaper-diaper-containing training pants which are used for teaching ayoung child when to use the toilet.

The water-absorbent resin composition has sufficient fluid passing spacenecessary for the movement of an aqueous fluid into the water-absorbentresin under pressure even in a so-called high dense condition, i.e.,even when the proportion of the water-absorbent resin composition to thetotal amount of the water-absorbent resin composition and the fibrousmaterial is not lower than 50 weight percent. Thus, the water-absorbentresin composition can achieve excellent permeability and dispersion ofthe aqueous fluid under pressure, and can improve the absorption rateand water retention capacity by capillarity without causing a gelblocking phenomenon. Consequently, even in the use in which the aqueousfluid is required to be absorbed a plurality of times, for example, whenthe absorbent article is a sanitary material, it is possible to preventthe sanitary material from leakage. Moreover, a reduction in thethickness of the sanitary material can be achieved.

For a fuller understanding of the nature and advantages of theinvention, reference should be made to the ensuing detailed descriptiontaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross sectional view of a measurement device usedfor measuring the absorbent capacity under pressure that is one of theperformances exhibited by a water-absorbent resin of the presentinvention.

FIG. 2 is a schematic cross sectional view of a measurement device usedfor measuring the liquid permeability-that is one of the performancesexhibited by a water-absorbent resin composition of the presentinvention.

FIG. 3 is a chart of ¹H-NMR of 2,2′-azobis(2-methylpropionamidine)dicarylate as a blowing agent of the present invention.

FIG. 4 is a chart of infrared absorption spectrum (IR) of the2,2′-azobis (2-methyl propionamidine) dicarylate.

FIG. 5 is a photograph as a drawing, indicating the structure ofparticles of a water-absorbent resin obtained in Example 11.

FIG. 6 is an electron photomicrograph as a drawing, indicating thestructure of the profile of a water-absorbent resin obtained in Example14.

FIG. 7 is a photograph as a drawing showing the structure of particlesof a water-absorbent resin obtained in Comparative Example 1.

BEST MODE FOR CARRYING OUT THE INVENTION

The following examples and comparative examples are provided to describethe present invention in greater detail, and are not meant to limit thepresent invention. Unless otherwise specified, “part” and “percent” asused in the following descriptions refer to “weight part” and “weightpercent”, respectively.

The performances of a water-absorbent resin or a water-absorbent resincomposition were measured by the following methods. When measuring theperformances, a water-absorbent resin in particle form was used. Morespecifically, the particle distribution of the water-absorbent resin wasadjusted so that particles having particle diameters ranging from 850 μmto 500 μm are 25 percent to 35 percent, particles having a particlediameters ranging rom 500 μm to 150 μm are 65 percent to 75 percent, andparticles having a particle diameters ranging from 150 μm to 10 μm are 0percent to 10 percent when the total amount is 100 percent.

In addition, a method for producing 2,2′-azobis(2-methylpropionamidine)dicarylate as an acrylic acid salt (blowing agent) of anazo-compound containing an amino group represented by general formula(1) above will be discussed below.

(1) Water Retention Capacity of Water-absorbent Resin

0.2 grams of water-absorbent resin was evenly placed in a tea bag (6cm×6 cm), and the tea bag was sealed by heat. The tea bag was thenplaced in an aqueous 0.9 percent sodium chloride solution (physiologicsaline). The tea bag was removed from the aqueous solution 60 minuteslater, and centrifuged at 1,300 rpm for three minutes using acentrifugal separator. The weight W_(1a) (g) of the tea bag wasmeasured. The same test was performed using an empty bag containing nowater-absorbent resin, and the weight W_(0a) (g) of the empty bag wasmeasured. The water retention capacity (gig) was calculated from theweights W_(1a) and W_(0a) according to the following equation.

Water retention capacity(g/g)

=(W_(1a) (g)−W(W_(0a)(g))/water-absorbent resin weight (g)

(2) Residual Monomer Content in Water-absorbent Resin

After placing 100 ml of deionized water in a 200 ml beaker, thedeionized water was completely gelated by adding 1.0 gram ofwater-absorbent resin while agitating. One hour later, 5 ml of aqueousphosphoric acid solution was added to the resulting gel to condensatethe gel. The condensed gel was filtered with a filter paper whileagitating, and the filtrate, i.e., water produced by condensation, wasanalyzed using a high-speed liquid chromatography.

Similarly, an aqueous monomer solution with a known concentration wasanalyzed as a standard solution to obtain a calibration curve. Bysetting the calibration curve as an external standard, the residualmonomer content (ppm) of the water-absorbent resin was calculated whileconsidering the dilution of the filtrate. Here, the residual monomercontent is given by a reduced value corresponding to the solid componentin the water-absorbent resin.

(3) Water-soluble Component Content in

Water-Absorbent Resin

0.5 grams of water-absorbent resin was dispersed in 1,000 ml ofdeionized water, agitated for 16 hours, and filtered with a filterpaper. The amount (percent) of water-soluble component was obtained bycolloid-titrating the resulting filtrate.

(4) Dispersion Rate (Dispersibility) of

Water-absorbent Resin

Various reagents were first dissolved in water to prepare aqueoussolutions containing 600 ppm to 700 ppm sodium cation, 65 ppm to 75 ppmcalcium cation, 55 ppm to 65 ppm magnesium cation, 1,100 ppm to 1,200ppm potassium cation, 240 ppm to 280 ppm phosphorus, 450 ppm to 500 ppmsulfur, 1,100 ppm to 1,300 ppm chlorine, and 1,300 ppm to 1,400 ppmsulfate group, respectively. These aqueous solutions were used assynthetic urine.

Next, 1.0 gram of water-absorbent resin was evenly spread in a Petridish with an internal diameter of 58 mm and a depth of 12 mm.Subsequently, 20 grams of synthetic urine having a temperature of 25° C.was calmly pored into the center of the Petri dish at a time. The timetaken from the initiation of pouring of synthetic urine to the time atwhich absorption of all the synthetic urine by the water-absorbent resinwas confirmed by eyes, was measured. The measured time was taken as adispersion rate (seconds).

(5) Dry Touch of Water-absorbent Resin

20 pieces of filter paper with a diameter of 55 mm were layered on thewater-absorbent resin after measuring the dispersion rate, i.e., on thewater-absorbent resin which absorbed the synthetic urine and swelled.The weight of the filter paper was measured in advance. A 500-gramweight (load) was placed on the filter paper, and the filter paper wasleft for one minute. The dry touch was evaluated by measuring the weightof the filter paper after the one minute leave and calculating anincrease (g) in the weight. Namely, as the amount of synthetic urinemoved from the swelled water-absorbent resin to the filter paper becomessmaller, the increase in the weight of the filter paper is smaller.Moreover, as the increase in the weight is smaller, the touch of theswelled water-absorbent resin becomes drier. Namely, it can be evaluatedthat the dry touch of the water-absorbent resin is excellent.

(6) Absorbent Capacity of Water-absorbent

Resin under Pressure

A measuring device for use in measuring the absorbent capacity underpressure is first explained briefly with reference to FIG. 1.

As illustrated in FIG. 1, the measuring device includes a scale 1, acontainer 2 of a predetermined capacity placed on the scale 1, anoutside air inlet pipe 2, a tube 4 made of a silicone resin, a glassfilter 6, and a measuring section 5 placed on the glass filter 6. Thecontainer 2 has an opening section 2 a at the top, and an opening 2 b onthe side section thereof. The outside air inlet pipe 3 was fitted intothe opening section 2 a, and the tube 4 was attached to the opening 2 b.The container 2 contains a predetermined amount of synthetic urine 12.The bottom end of the outside air inlet pipe 3 sinks in the syntheticurine 12. The outside air inlet pipe 3 was provided to keep the pressurein the container 2 at substantially atmospheric pressure. The glassfilter 6 was formed to have a diameter of 55 mm. The container 2 and theglass filter 6 are connected to each other with the tube 4. The positionand height of the glass filter 6 with respect to the container 2 wasfixed.

The measuring section 5 includes filter paper 7, a bearing cylinder 9, ametal gauge 10 attached to the bottom of the bearing cylinder 9, and aweight 11. In the measuring section 5, the paper filter 7 and thebearing cylinder 9 (i.e., the metal gauge 10) are placed in this orderon the glass filter 6, and the weight 11 is placed inside the bearingcylinder 9, i.e., on the metal gauge 10. The metal gauge 10 was made ofstainless steel and is 400 mesh (38 μm in mesh). A predetermined amountof water-absorbent resin 15 having a predetermined particle diameter wasevenly spread over the metal gauge 10 in measuring. In addition, the topsurface of the metal gauge 10, i.e., the contact surface between themetal gauge 10 and the water-absorbent resin 15, was arranged at thesame level as the height of a lower end surface 3 a of the outside airinlet pipe 3. The weight of the weight 11 was adjusted so that a load of50 g/cm² was evenly applied to the metal gauge 10, i.e., to thewater-absorbent resin 15.

The absorbent capacity under pressure was measured using a measuringdevice of the above-mentioned structure. The following description willdiscuss the measuring method.

First, prescribed preparations were made. For example, a predeterminedamount of the synthetic urine 12 was placed in the container 2, and theoutside air inlet pipe 3 was fitted into the container 2. Next, thefilter paper 7 was placed on the glass filter 6. At the same time asplacing the filter paler 7 on the glass filter 6, 0.9 grams ofwater-absorbent resin was evenly spread inside the bearing cylinder 9,i.e., on the metal gauge 10, and the weight 11 was then placed on thewater-absorbent resin 15.

Subsequently, the metal gauge 10, i.e., the bearing cylinder 9 whereuponthe water-absorbent resin 15 and the weight 11 were placed, was placedon the filter paper 7 so that the center of the bearing cylinder 9coincides with the center of the glass filter 6.

Weight W₂(g) of the synthetic urine 12 absorbed by the water-absorbentresin 15 was measured using the scale 1 with the passage of time over 60minutes after the placement of the bearing cylinder 9 on the filterpaper 7. The same process was performed without using thewater-absorbent resin 15, and the weight, i.e., the weight of syntheticurine 12 absorbed by members other than the water-absorbent resin 15,for example, the filter paper 7, was measured as blank weight W₃(g) withthe scale 1. The absorbent capacity under pressure (g/g) was calculatedfrom the weights W₂ and W₃ according to the following equation.

Absorbent capacity under pressure (g/g)

=(W₂(g)−W₃ (g))/water-absorbent resin weight(g)

(7) Water Retention Capacity of

Water-absorbent Resin Composition

0.2 grams of water-absorbent resin composition was evenly placed in atea bag (6 cm×6 cm), and the tea bag was sealed by heat. The tea bag wasthen placed in an aqueous 0.9 percent sodium chloride solution(physiologic saline). 60 minutes later, the tea bag was removed from thesolution and centrifuged at 250G for three minutes using a centrifugalseparator. The weight W_(1b) (g) of the tea bag was measured. The sametest was performed using an empty bag containing no water-absorbentresin composition, and the weight W_(0b) (g) of the empty bag wasmeasured. The water retention capacity (g/g) was calculated from theweights W_(1b) and W_(0b) according to the following equation. Waterretention capacity(g/g)

=(W_(1b)(g)−W_(0b)(g))/water-absorbent resin composition weight (g)

(8) Absorption Rate of Water-absorbent

Resin Composition

1.0 gram of water-absorbent resin composition was placed in apolypropylene cylindrical cup having an internal diameter of 50 mm and aheight of 70 mm. Subsequently, 28 grams of physiologic saline was poredinto the cup. The time taken for reaching a state in which thephysiologic saline was completely absorbed by the water-absorbent resincomposition and become unseen from the initiation of pouring of thephysiologic saline was measured. The measurement was performed threetimes, and the average value was taken as the absorption rate (seconds).

(9) Rate of Fluid to Flow through Water-absorbent Resin

Composition (Liquid Permeability) Under Pressure

A measuring device for use in measuring the liquid permeability underpressure will be briefly explained with reference to FIG. 2.

As illustrated in FIG. 2, the measuring device includes a glass column20, a pressure shaft 21, and a weight 22. The glass column 20 is formedinto a cylindrical shape with an internal diameter of one inch and aheight of 400 mm. Attached below the glass column 20 is a cock 25 whichis freely opened and closed. Moreover, a glass filter 27 is insertedinto the glass column 20 so as to fill the glass column 20 with awater-absorbent resin composition 30. The mesh of the glass filter 27 is#G2. The glass column 20 is provided with standard lines L and M. Thestandard line L is given at a level 150 mm distant from the top surfaceof the glass filter 27. The standard line M is given at a level 100 mmdistant from the top surface of the glass filter 27. The glass column 20contains a predetermined amount of physiologic saline 29 therein.BIO-COLUMN CF-30K (trade name, catalog code: 22-635-07, produced byKabushiki Kaisha Iuchi Seieido) was used as the glass column 20.

A plate 23 on which the weight 22 is placable is fixed to the upper endsection of the pressure shaft 21. The plate 23 is shaped into a circularplate of a diameter slightly smaller than the internal diameter of theglass column 20. The pressure shaft 21 has a length that prevents theplate 23 from sinking in the physiologic saline 29.

Moreover, a pressure plate 24 is fixed to the lower end section of thepressure shaft 21. The pressure plate 24 is shaped into a circular platewith a diameter of about 1 inch and a thickness of 10 mm, and has 64holes 24 a on the upper surface through the lower surface. The holes 24a have a diameter of 1 mm and positioned at an interval of about 2 mm.In this structure, the physiologic saline 29 flows from the uppersurface to the lower surface of the pressure plate 24 through the holes24 a.

The pressure shaft 21, i.e., the pressure plate 24, is movable in upwardand downward directions in the glass column 20. A glass filter 26 isattached to the lower surface of the pressure plate 24. The mesh of theglass filter 26 is #G0.

The weight of the weight 22 is adjusted so as to allow a load of 24.5g/cm² to be evenly applied to the swelled water-absorbent resincomposition 30.

The liquid permeability was measured with a measuring device having theabove-mentioned structure. The measuring method will be discussed below.

First, the cock 25 is closed, and the glass filter 27 is inserted intothe glass column 20. 0.5 grams of water-absorbent resin composition isthen placed in the glass column 20. Next, an amount of the physiologicsaline 29 that could not be absorbed by the water-absorbent resincomposition 30, i.e., an excessive amount of physiologic saline 29, isplaced in the glass column 20 to swell the water-absorbent resincomposition 30.

About one hour later, the water-absorbent resin composition 30sufficiently settles out and the swelling thereof reaches an equilibriumcondition. Then, the pressure shaft 21 is inserted into the glass column20. More specifically, the pressure plate 24 is placed on thewater-absorbent resin composition 30 while discharging the air so thatthe air does not remain between the swelled water-absorbent resincomposition 30 and the glass filter 26. Thereafter, the weight 22 isplaced on the plate 23 to press the water-absorbent resin composition30.

Subsequently, the physiologic saline 29 is added to adjust the height(fluid depth) H from the upper surface of the glass filter 27 (thelowest section of the water-absorbent resin composition 30) to theliquid level to be 200 mm.

Next, the cock 25 is opened to discharge the physiologic saline 29, andthe time between a passage of the liquid level of the physiologic saline29 through the standard line L and a passage of the liquid level throughthe standard line M is measured. The amount of the physiologic saline 29discharged during the measured time is about 25 ml (observed value).

The measurement is performed three times, and the average value is takenas the rate of fluid to flow “liquid permeability” (seconds). Themeasurement is also carried out in the same manner without using thewater-absorbent resin composition 30. In this case, the liquidpermeability was 10 seconds.

(10) Method for Producing 2,2′-azobis(2-methyl propionamidine)diacrylate

6.7 parts aqueous 37 percent sodium acrylate solution as acrylic acidsalt was added to 36 parts aqueous 10 percent 2,2′-azobis(2-methylpropionamidine)dihydrochloride solution as a blowing agent precursorwhich was kept at 20 ° C. while agitating the aqueous2,2′-azobis(2-methyl propionamidine)dihuydrochloride solution at 1,200rpm. Several seconds after the addition, the aqueous solution appearedcloudy or white, and white fine solid particles with an average particlediameter of 10 μm were generated. The fine solid particles were evenlydispersed in the aqueous solution.

About 2.2 parts fine solid particles were isolated by filtering theaqueous solution, and then purified by washing with water. Theultraviolet absorption spectrum (UV) of the resulting fine solidparticles was measured. As a result, absorption unique to azo groups wasobserved at 365 nm. Additionally, elemental analysis of the fine solidparticles was performed, and ¹H-NMR (nuclear magnetic resonance) andinfrared absorption spectrum (IR) were measured. In the measurement of¹H-NMR, heavy water was used as a solvent.

As a result, it was confirmed that the fine solid particles were2,2′-azobis(2-methyl propionamidine)diacrylate as an acrylic acid saltof an azo-compound containing an amino group (blowing agent),represented by general formula (1) mentioned above. The chart of ¹H-NMRand the chart of IR are shown in FIGS. 3 and 4, respectively.

EXAMPLE 1

First, 38.6 parts acrylic acid as unsaturated monomer, 409 parts aqueous37 percent sodium acrylate solution, 0.48 parts trimethylolpropanetriacrylate as a cross-linking agent, and 53 parts deionized water weremixed to prepare an aqueous monomer solution. Namely, the aqueousmonomer solution is an aqueous 38 percent acrylate monomer solution witha neutralization rate of 75 mole percent.

By bubbling a nitrogen gas into the aqueous monomer solution whilekeeping the temperature of the aqueous monomer solution at 25° C.,dissolved oxygen was removed. Next, 4.3 parts 10 percent aqueous2,2′-azobis(2-methyl propionamidine)dihydrochloride solution as ablowing agent precursor was added while agitating the aqueous monomersolution. Thereafter, the aqueous solution was agitated at 25° C. underthe flow of nitrogen.

About seven minutes after the initiation of agitation, the aqueoussolution appeared cloudy or white, and white fine solid particles withan average particle diameter of 9 μm were generated. The fine solidparticles was 2,2′-azobis(2-methyl propionamidine)diacrylate as ablowing agent. Moreover, 10 minutes after the initiation of agitation,the solid content in the aqueous monomer solution, i.e., the amount of2,2′-azobis(2-methyl propionamidine)diacrylate generated became 0.29percent based on the acrylate monomer. The 2,2′-azobis(2-methylpropionamidine)diacrylate was uniformly dispersed in the aqueous monomersolution.

At this time (10 minutes after the initiation of agitation), 2.6 partsaqueous 10 percent sodium persulfate solution and 1 part aqueous 1percent L-ascorbic acid solution were added as a redox initiator(radical polymerization initiator) while agitating the aqueous monomersolution. After sufficiently agitating the aqueous monomer solution, itwas left at rest.

About 10 minutes after the addition of aqueous sodium persulfatesolution, the temperature of the aqueous monomer solution reached about89° C. Thereafter, the aqueous monomer solution was left at rest forfurther 10 minutes while keeping the temperature thereof between 70° C.and 80° C. to polymerize the acrylate monomer. As a result, a hydrogelhaving cells as a porous cross-linking polymer was obtained.

The resulting hydrogel having cells was removed, made into small piecesof a size ranging from about 20 mm to 1 mm, and hot-air dried at 150° C.with a hot-air drier. Subsequently, the dried pieces were ground with aroll mill, and sieved using a standard screen (850 μm) according to JISstandards, thereby producing a water-absorbent resin of the presentinvention.

It was confirmed through an electron photomicrograph that theabove-mentioned water-absorbent resin was porous. The average porediameter of the water-absorbent resin was 60 μm. Moreover, variousphysical properties of the water-absorbent resin were measured by theabove-mentioned methods. The water retention capacity was 29 g/g, theresidual monomer content was 200 ppm, the water-soluble componentcontent was 9 percent, the dispersion rate was 33 seconds, the dry touchwas 4.3 g, and the absorbent capacity under pressure was 11 g/g. Theresults are shown in Table 1.

EXAMPLE 2

First, 38.6 parts acrylic acid, 409 parts aqueous 37 percent sodiumacrylate solution, 1.08 parts polyethylene glycol diacrylate as across-linking agent, and 53 parts deionized water were mixed to preparean aqueous monomer solution. Namely, the aqueous monomer solution is anaqueous 38 percent acrylate monomer solution with a neutralization rateof 75 mole percent.

By bubbling a nitrogen gas into the aqueous monomer solution whilekeeping the temperature of the aqueous monomer solution at 25° C.,dissolved oxygen was removed. Next, 4.3 parts aqueous 10 percent2,2′-azobis(2-methyl propionamidine)dihydrochloride solution was addedwhile agitating the aqueous monomer solution. Thereafter, the aqueoussolution was agitated at 25° C. under the flow of nitrogen.

About seven minutes later from the initiation of agitation, the aqueoussolution appeared cloudy or white, and 2,2′-azobis(2-methylpropionamidine)diacrylate in the form of white fine particles with anaverage particle diameter of 9 μm was generated. 10 minutes later fromthe initiation of agitation, the amount of 2,2′-azobis (2-methylpropionamidine)diacrylate generated became 0.29 percent based on theacrylate monomer. The 2,2′-azobis(2-methyl propionamidine)diacrylate wasevenly dispersed in the aqueous monomer solution.

At this time, 2.6 parts aqueous 10 percent sodium persulfate solutionand 1 part aqueous 1 percent L-ascorbic acid solution were added whileagitating the aqueous monomer solution. After the addition, the aqueousmonomer solution continued to be agitated.

About 10 minutes after the addition of aqueous sodium persulfatesolution, the temperature of the aqueous monomer solution reached about79° C. Thereafter, the aqueous monomer solution was further agitated for10 minutes while keeping the temperature thereof between 70° C. and 80°C. to polymerize the acrylate monomer. As a result, a hydrogel havingcells was obtained.

The resulting hydrogel having cells was removed, and the same operationsas in Example 1 were performed to produce a water-absorbent resin. Theaverage pore diameter of the water-absorbent resin was 70 μm. Moreover,various physical properties of the water-absorbent resin were measuredby the above-mentioned methods. The water retention capacity was 33 g/g,the residual monomer content was 170 ppm, the water-soluble componentcontent was 6 percent, the dispersion rate was 30 seconds, the dry touchwas 3.9 g, and the absorbent capacity under pressure was 12 g/g. Theresults are shown in Table 1.

EXAMPLE 3

An aqueous monomer solution was prepared in the same manner as inExample 2. By bubbling a nitrogen gas into the aqueous monomer solutionwhile keeping the temperature of the aqueous monomer solution at 25° C.,dissolved oxygen was removed. Next, 21 parts aqueous 10 percent2,2′-azobis (2-methyl propionamidine) dihydrochloride solution was addedwhile agitating the aqueous monomer solution. Thereafter, the aqueoussolution was agitated at 25° C. under the flow of nitrogen.

About one minute later from the initiation of agitation, the aqueoussolution appeared cloudy or white, and 2,2′-azobis(2-methylpropionamidine)diacrylate in the form of white fine particles with anaverage particle diameter of 10 gm was generated. Five minutes laterfrom the initiation of agitation, the amount of 2,2′-azobis(2-methylpropionamidine)diacrylate generated became 1.4 percent based on theacrylate monomer. The 2,2′-azobis(2-methyl propionamidine)diacrylate wasevenly dispersed in the aqueous monomer solution.

At this time, 2.6 parts aqueous 10 percent sodium persulfate solutionand 1 part aqueous 1 percent L-ascorbic acid solution were added whileagitating the aqueous monomer solution. After the addition, the aqueousmonomer solution continued to be agitated.

About seven minutes after the addition of aqueous sodium persulfatesolution, the temperature of the aqueous monomer solution reached about82° C. Thereafter, the aqueous monomer solution was further agitated for10 minutes while keeping the temperature thereof between 70° C. and 80°C. to polymerize the acrylate monomer. As a result, a hydrogel havingcells was obtained.

The resulting hydrogel having cells was removed, and the same operationsas in Example 1 were performed to produce a water-absorbent resin. Theaverage pore diameter of the water-absorbent resin was 70 μm. Moreover,various physical properties of the water-absorbent resin were measuredby the above-mentioned methods. The water retention capacity was 27 g/g,the residual monomer content was 120 ppm, the water-soluble componentcontent was 5 percent, the dispersion rate was 37 seconds, the dry touchwas 4.1 g, and the absorbent capacity under pressure was 11 g/g. Theresults are shown in Table 1.

EXAMPLE 4

First, 18 parts acrylic acid, 190 parts aqueous 37 percent sodiumacrylate solution, 0.154 parts N,N′-methylenebisacrylamide as across-linking agent, and 21 parts deionized water were mixed to preparean aqueous monomer solution. Namely, the aqueous monomer solution is anaqueous 38 percent acrylate monomer solution with a neutralization rateof 75 mole percent.

By bubbling a nitrogen gas into the aqueous monomer solution whilekeeping the temperature of the aqueous monomer solution at 25° C.,dissolved oxygen was removed. Next, 1.34 parts powdered2,2′-azobis(2-methyl propionamidine)diacrylate prepared by theabove-mentioned production method was added while agitating the aqueousmonomer solution. Thereafter, the aqueous solution was agitated at 25°C. under the flow of nitrogen. The 2,2′-azobis(2-methylpropionamidine)diacrylate was evenly dispersed in the aqueous monomersolution.

Next, 1.2 parts aqueous 10 percent sodium persulfate solution and 0.5parts aqueous 1 percent L-ascorbic acid solution were added whileagitating the aqueous monomer solution. After the addition, the aqueousmonomer solution continued to be agitated.

About 13 minutes after the addition of aqueous sodium persulfatesolution, the temperature of the aqueous monomer solution reached about92° C. Thereafter, the aqueous monomer solution was further agitated forone hour while keeping the temperature thereof between 60° C. and 80° C.to polymerize the acrylate monomer. As a result, a hydrogel having cellswas obtained.

The resulting hydrogel having cells was removed, and the same operationsas in Example 1 were performed to produce a water-absorbent resin. Theaverage pore diameter of the water-absorbent resin was 65 μm. Moreover,various physical properties of the water-absorbent resin were measuredby the above-mentioned methods. The water retention capacity was 27 g/g,the residual monomer content was 220 ppm, the water-soluble componentcontent was 9 percent, the dispersion rate was 25 seconds, the dry touchwas 4.3 g, and the absorbent capacity under pressure was 9 g/g. Theresults are shown in Table 1.

EXAMPLE 5

First, 21.6 parts acrylic acid, 178 parts aqueous 37 percent sodiumacrylate solution, 0.046 parts N,N′-methylenebisacrylamide, 0.18 partshydroxylethyl cellulose as water-soluble polymer (dispersionstabilizer), and 50 parts deionized water were mixed to prepare anaqueous monomer solution. Namely, the aqueous monomer solution is anaqueous 38 percent acrylate monomer solution with a neutralization rateof 70 mole percent.

By bubbling a nitrogen gas into the aqueous monomer solution whilekeeping the temperature of the aqueous monomer solution at 25° C.,dissolved oxygen was removed. Subsequently, 0.18 parts polyoxyethylenesorbitan monostearate as a surface active agent (dispersion stabilizer),and 2.63 parts ground calcium carbonate as a blowing agent were addedwhile agitating the aqueous monomer solution. The calcium carbonate hadan average particle diameter of 3 μm, and was evenly dispersed in theaqueous monomer solution.

Next, 1.2 parts aqueous 10 percent sodium persulfate solution and 0.5parts aqueous 1 percent L-ascorbic acid solution were added whileagitating the aqueous monomer solution at 25° C. under the flow ofnitrogen. After sufficiently agitating the aqueous monomer solution, itwas left at rest.

About 10 minutes after the addition of aqueous sodium persulfatesolution, the temperature of the aqueous monomer solution reached about99° C. Thereafter, the aqueous monomer solution was further agitated for10 minutes while keeping the temperature thereof between 60° C. and 80°C. to polymerize the acrylate monomer. As a result, a hydrogel havingcells was obtained.

The resulting hydrogel having cells was removed, and the same operationsas in Example 1 were performed to produce a water-absorbent resin. Theaverage pore diameter of the water-absorbent resin was 250 μm. Moreover,various physical properties of the water-absorbent resin were measuredby the above-mentioned methods. The water retention capacity was 45 g/g,the residual monomer content was 520 ppm, the water-soluble componentcontent was 13 percent, the dispersion rate was 24 seconds, the drytouch was 4.5 g, and the absorbent capacity under pressure was 8 g/g.The results are shown in Table 1.

COMPARATIVE EXAMPLE 1

An aqueous monomer solution was prepared in the same manner as inExample 1. By bubbling a nitrogen gas into the aqueous monomer solutionwhile keeping the temperature of the aqueous monomer solution at 25° C.,dissolved oxygen was removed. Next, 4.3 parts aqueous 10 percent2,2′-azobis (2-methyl propionamidine)dihydrochloride solution was addedto the aqueous monomer solution, and dissolved therein. Thereafter, 2.6parts aqueous 10 percent sodium persulfate solution and 1 part aqueous 1percent L-ascorbic acid solution were immediately added while agitatingthe aqueous monomer solution. After the addition, the aqueous monomersolution continued to be agitated, and was left at rest upon theinitiation of polymerization. Namely, the acrylate monomer waspolymerized without using the blowing agent of the present invention.

About 10 minutes after the addition of aqueous sodium persulfatesolution, the temperature of the aqueous monomer solution reached about95° C. Thereafter, the aqueous monomer solution was further agitated for10 minutes while keeping the temperature thereof between 70° C. and 85°C. to polymerize the acrylate monomer. As a result, a hydrogel havingsubstantially no cell was obtained. The hydrogel had a few cells rangingfrom 2 mm to 4 mm in size.

The resulting hydrogel was removed, and the same operations as inExample 1 were performed to produce a comparative water-absorbent resin.The comparative water-absorbent resin had no pores. Moreover, variousphysical properties of the comparative water-absorbent resin weremeasured by the above-mentioned methods. The water retention capacitywas 29 g/g, the residual monomer content was 540 ppm, the water-solublecomponent content was 14 percent, the dispersion rate was 63 seconds,the dry touch was 6.1 g, and the absorbent capacity under pressure was 7g/g. Thus, the comparative water-absorbent resin had declined dispersionrate and dry touch. The results are shown in Table 1. FIG. 7 shows anelectron photomicrograph indicating the particle structure of thecomparative water-absorbent resin having a particle diameter rangingfrom 300 μm to 600 μm.

COMPARATIVE EXAMPLE 2

First, 21.6 parts acrylic acid, 178 parts aqueous 37 percent sodiumacrylate solution, 0.046 parts N,N′-methylenebisacrylamide, and 50 partsdeionized water were mixed to prepare an aqueous monomer solution. Bybubbling a nitrogen gas into the aqueous monomer solution while keepingthe temperature of the aqueous monomer solution at 25° C., dissolvedoxygen was removed. Next, 2.63 parts sodium carbonate as a blowing agentwas added while agitating the aqueous monomer solution.

As a result, since carbon dioxide gas was generated, 1.2 parts aqueous10 percent sodium persulfate solution and 0.5 parts aqueous 1 percentL-ascorbic acid solution were immediately added while agitating theaqueous monomer solution. After the addition, the aqueous monomersolution continued to be agitated. In this case, calcium carbonate wasnot dispersed in the aqueous monomer solution.

About 10 minutes after the addition of aqueous sodium persulfatesolution, the temperature of the aqueous monomer solution reached about97° C. Thereafter, the aqueous monomer solution was further agitated for10 minutes while keeping the temperature thereof between 60° C. and 80°C. to polymerize the acrylate monomer. As a result, a hydrogel havingcells was obtained.

The resulting hydrogel having cells was removed, and the same operationsas in Example 1 were performed to produce a comparative water-absorbentresin. The average pore diameter of the comparative water-absorbentresin was about 600 μm. Moreover, various physical properties of thecomparative water-absorbent resin were measured by the above-mentionedmethods. The water retention capacity was 40 g/g, the residual monomercontent was 3,400 ppm, the water-soluble component content was 17percent, the dispersion rate was 47 seconds, the dry touch was 6.5 g,and the absorbent capacity under pressure was 7 g/g. Thus, thecomparative water-absorbent resin have increased residual monomer andwater-soluble component contents, and declined dispersion rate and drytouch. The results are shown in Table 1.

EXAMPLE 6

The water-absorbent resin obtained in Example 1 was subjected tosecondary cross-linking treatment. First, 0.05 parts ethylene glycoldiglycidyl ether and 0.75 parts glycerin as surface cross-linkingagents, 0.5 parts lactic acid as a mixing assistant, 3 parts water as ahydrophilic solvent, and 0.75 parts isopropyl alcohol were mixed toprepare a mixed solution.

Next, 100 parts water-absorbent resin produced in Example 1 and themixed solution were combined, and the resulting mixture was heated at195° C. for 20 minutes. As a result, a water-absorbent resin having acovalent bond and improved cross-link density in the vicinity ofsurface, i.e., a water-absorbent resin which underwent the secondarycross-linking treatment, was obtained. The average pore diameter of thewater-absorbent resin was 60 μm. Moreover, various physical propertiesof the water-absorbent resin were measured by the above-mentionedmethods. The water retention capacity was 27 g/g, the residual monomercontent was 180 ppm, the water-soluble component content was 9 percent,the dispersion rate was 28 seconds, the dry touch was 3.4 g, and theabsorbent capacity under pressure was 30 g/g. The results are shown inTable 1.

EXAMPLE 7

The water-absorbent resin obtained in Example 2 was subjected tosecondary cross-linking treatment. First, 1 part glycerin, 3 partswater, and 1.75 parts isopropyl alcohol were mixed to prepare a mixedsolution.

Next, 100 parts water-absorbent resin produced in Example 2 and themixed solution were combined, and the resulting mixture was heated at195° C. for 25 minutes. As a result, a water-absorbent resin having acovalent bond and improved cross-link density in the vicinity ofsurface, i.e., a water-absorbent resin which underwent the secondarycross-linking treatment, was obtained. The average pore diameter of thewater-absorbent resin was 70 μm. Moreover, various physical propertiesof the water-absorbent resin were measured by the above-mentionedmethods. The water retention capacity was 30 g/g, the residual monomercontent was 160 ppm, the water-soluble component content was 6 percent,the dispersion rate was 30 seconds, the dry touch was 2.9 g, and theabsorbent capacity under pressure was 31 gig. The results are shown inTable 1.

EXAMPLE 8

The water-absorbent resin obtained in Example 3 was subjected tosecondary cross-linking treatment. First, 0.05 parts ethylene glycoldiglycidyl ether, 0.75 parts glycerin, 0.5 parts polyaspartic acid as amixing assistant, 3 parts water, and 5 parts isopropyl alcohol weremixed to prepare a mixed solution.

Next, 100 parts water-absorbent resin produced in Example 3 and themixed solution were combined, and the resulting mixture was heated at195° C. for 15 minutes. As a result, a water-absorbent resin having acovalent bond and improved cross-link density in the vicinity ofsurface, i.e., a water-absorbent resin which underwent the secondarycross-linking treatment, was obtained. The average pore diameter of thewater-absorbent resin was 70 μm. Moreover, various physical propertiesof the water-absorbent resin were measured by the above-mentionedmethods. The water retention capacity was 26 g/g, the residual monomercontent was 120 ppm, the water-soluble component content was 5 percent,the dispersion rate was 35 seconds, the dry touch was 3.3 g, and theabsorbent capacity under pressure was 29 g/g. The results are shown inTable 1.

EXAMPLE 9

The water-absorbent resin obtained in Example 6 was further subjected tocross-linking treatment. More specifically, 100 parts water-absorbentresin produced in Example 6 and 5 parts aqueous 30 percent solution ofpolyethylene imine having an average molecular weight of 70,000 wascombined, and the resulting mixture was heated. As a result, awater-absorbent resin having an ionic bond and further improvedcross-link density in the vicinity of surface was obtained. Theresulting water-absorbent resin had further improved physical propertiesthan the water-absorbent resin before the treatment.

The water-absorbent resins obtained in Examples 7 and 8 also underwentthe above-mentioned treatment. The resulting water-absorbent resins hadfurther improved physical properties than the water-absorbent resinsbefore the treatment.

COMPARATIVE EXAMPLE 3

The comparative water-absorbent resin obtained in Comparative Example 2was subjected to secondary cross-linking treatment. First, 0.05 partsethylene glycol diglycidyl ether, 0.75 parts glycerin, 0.5 parts lacticacid, 3 parts water, and 0.75 parts isopropyl alcohol were mixed toprepare a mixed solution.

Next, 100 parts comparative water-absorbent resin produced inComparative Example 2 and the mixed solution were combined, and theresulting mixture was heated at 195 ° C. for 20 minutes. As a result, acomparative water-absorbent resin having a covalent bond and improvedcross-link density in the vicinity of surface, i.e., a comparativewater-absorbent resin which underwent the secondary cross-linkingtreatment, was obtained. The average pore diameter of the comparativewater-absorbent resin was about 600 μm. Moreover, various physicalproperties of the comparative water-absorbent resin were measured by theabove-mentioned methods. The water retention capacity was 35 g/g, theresidual monomer content was 3,400 ppm, the water-soluble componentcontent was 17 percent, the dispersion rate was 40 seconds, the drytouch was 5.5 g, and the absorbent capacity under pressure was 23 g/g.Thus, the resulting comparative resin had increased residual monomer andwater-soluble component contents, and declined dispersion rate and drytouch. The results are shown in Table 1.

TABLE 1 Water Water- absorbent Water Residual soluble capacity Averageretention monomer component Dispersion Dry under pore capacity contentcontent rate touch pressure diameter (g/g) (ppm) (percent) (second) (g)(g/g) (μm) Examples 1 29 200 9 33 4.3 11 60 2 33 170 6 30 3.9 12 70 3 27120 5 37 4.1 11 70 4 27 220 9 25 4.3  9 65 5 45 520 13  24 4.5  8 250  627 180 9 28 3.4 30 60 7 30 160 6 30 2.9 31 70 8 26 120 5 35 3.3 29 70Comparative examples 1 29 540 14  63 6.1  7 — 2 40 3400  17  47 6.5  7600  3 35 3400  17  40 5.5 23 600 

EXAMPLE 10

Dissolved oxygen was removed by bubbling a nitrogen gas into thesolution while keeping the temperature of 166 parts aqueous 37 percentsodium acrylate solution at 25° C. Next, 47 parts aqueous 10 percent2,2′-azobis(2-methyl propionamidine) dihydrochloride solution was addedwhile agitating the aqueous solution. Thereafter, the aqueous solutionwas agitated at 25° C. under the flow of nitrogen.

About one minute later from the initiation of agitation, the aqueoussolution appeared cloudy or white, and 2,2′-azobis(2-methylpropionamidine)diacrylate in the form of white fine particles with anaverage particle diameter of 15 μm was generated. The2,2′-azobis(2-methyl propionamidine)diacrylate was evenly dispersed inthe aqueous solution.

At this time (one minute later from the initiation of agitation), 425parts acrylic acid and 5.23 parts trimethylolpropane triacrylate weremixed and dissolved while agitating the aqueous solution to prepare anaqueous monomer solution in which the 2,2′-azobis(2-methylpropionamidine) diacrylate was evenly dispersed (hereinafter justreferred to as the aqueous monomer solution). Namely, the aqueousmonomer solution is an aqueous 38 percent acrylate monomer solutionhaving a neutralization rate of 75 mole percent.

Meanwhile, a 2L separable flask equipped with an agitator, a refluxcondenser, a thermometer, a dropping funnel and a nitrogen gas inlettube was used as a reaction container 4 grams of sucrose fatty acidester (trade name: DK-ESTER F-50, produced by Dai-ichi Kogyo SeiyakuLtd.) as a dispersion stabilizer, and cyclohexane 2 L as a solvent wereplaced in the reaction container. Moreover, 300 grams of aqueous monomersolution mentioned above was placed in the dropping funnel.

The aqueous monomer solution was dropped while agitating the cyclohexanesolution at 230 rpm to disperse and suspend the aqueous monomersolution. Next, the cyclohexane solution was agitated at 60° C. for twohours to perform reversed-phase suspension polymerization of theacrylate monomer. Thereafter, water generated by the reaction wasremoved by forming an azeotropic with cyclohexane (azeotropicdehydration). By filtering the cyclohexane solution, a water-absorbentresin in the spherical form with an average particle diameter of severalhundred μm, i.e., a water-absorbent resin of the present invention, wasobtained.

The average pore diameter of the water-absorbent resin was 50 μm.Moreover, various physical properties of the water-absorbent resin weremeasured by the above-mentioned methods. The water retention capacitywas 30 g/g, the residual monomer content was 70 ppm, the water-solublecomponent content was 9 percent, the dispersion rate was 40 seconds, thedry touch was 4.5 g, and the absorbent capacity under pressure was 10g/g. The results are shown in Table 2.

EXAMPLE 11

First, 216 parts acrylic acid, 4,321 parts aqueous 37 percent sodiumacrylate solution, 5.8 parts polyethylene glycol diacrylate, and 887parts water were mixed to prepare an aqueous monomer solution. Namely,the aqueous monomer solution is an aqueous 33 percent acrylate monomersolution having a neutralization rate of 85 mole percent.

Dissolved oxygen was removed by bubbling a nitrogen gas into thesolution while keeping the temperature of the aqueous monomer solutionat 25° C. Next, 40 parts aqueous 10 percent 2,2′-azobis (2-methylpropionamidine)dihydrochloride solution was added while agitating theaqueous monomer solution. Thereafter, the aqueous solution was agitatedat 25° C. under the flow of nitrogen.

About six minutes later from the initiation of agitation, the aqueoussolution appeared cloudy or white, and 2,2′-azobis(2-methylpropionamidine)diacrylate in the form of white fine particles with anaverage particle diameter of 7 μm was generated. The2,2′-azobis(2-methyl propion amidine)diacrylate was evenly dispersed inthe aqueous monomer solution.

At this time, 28 parts aqueous 10 percent sodium persulfate solution and1.3 parts aqueous 1 percent L-ascorbic acid solution were added whileagitating the aqueous monomer solution. After the addition, the aqueousmonomer solution continued to be agitated.

About 30 seconds after the addition of aqueous sodium persulfatesolution, polymerization of the acrylate monomer was initiated. As aresult, a hydrogel having cells was obtained.

The resulting hydrogel having cells was removed, and the same operationsas in Example 1 were performed to produce a water-absorbent resin. Theaverage pore diameter of the water-absorbent resin was 50 μm. Moreover,various physical properties of the water-absorbent resin were measuredby the above-mentioned methods. The water retention capacity was 39 g/g,the residual monomer content was 240 ppm, the water-soluble componentcontent was 8 percent, the dispersion rate was 21 seconds, the dry touchwas 4.0 g, and the absorbent capacity under pressure was 11 g/g. Theresults are shown in Table 2. FIG. 5 shows an electron photomicrographindicating the particle structure of a water-absorbent resin having aparticle diameter ranging from 300 μm to 600 μm.

EXAMPLE 12

First, 375 parts acrylic acid, 5,290 parts aqueous 37 percent sodiumacrylate solution, 6.3 parts polyethylene glycol diacrylate, and 808parts water were mixed to prepare an aqueous monomer solution. Namely,the aqueous monomer solution is an aqueous 35.5 percent acrylate monomersolution having a neutralization rate of 85 mole percent.

Dissolved oxygen was removed by bubbling a nitrogen gas into thesolution while keeping the temperature of the aqueous monomer solutionat 25° C. Next, 52 parts aqueous 10 percent 2,2 ′-azobis (2-methylpropionamidine) dihydrochloride solution was added while agitating theaqueous monomer solution. Thereafter, the aqueous solution was agitatedat 25° C. under the flow of nitrogen.

About 2.5 minutes later from the initiation of agitation, the aqueoussolution appeared cloudy or white, and 2,2′-azobis(2-methylpropionamidine)diacrylate in the form of white fine particles with anaverage particle diameter of 9 μm was generated. The2,2′-azobis(2-methyl propionamidine)diacrylate was evenly dispersed inthe aqueous monomer solution.

At this point, 36 parts aqueous 10 percent sodium persulfatesolution-and 1.7 parts aqueous 1 percent L-ascorbic acid solution wereadded while agitating the aqueous monomer solution. After the addition,the aqueous monomer solution continued to be agitated.

About 30 seconds after the addition of aqueous sodium persulfatesolution, polymerization of the acrylate monomer was initiated. As aresult, a hydrogel having cells was obtained.

The resulting hydrogel having cells was removed, and the same operationsas in Example 1 were performed to produce a water-absorbent resin. Theaverage pore diameter of the water-absorbent resin was 100 μm. Moreover,various physical properties of the water-absorbent resin were measuredby the above-mentioned methods. The water retention capacity was 38 g/g,the residual monomer content was 270 ppm, the water-soluble componentcontent was 9 percent, the dispersion rate was 24 seconds, the dry touchwas 4.0 g, and the absorbent capacity under pressure was 10 g/g. Theresults are shown in Table 2.

EXAMPLE 13

First, 375 parts acrylic acid, 5,290 parts aqueous 37 percent sodiumacrylate solution, and 6.3 parts polyethylene glycol diacrylate weremixed to prepare an aqueous monomer solution. Namely, the aqueousmonomer solution is an aqueous about 42 percent acrylate monomersolution having a neutralization rate of 85 mole percent.

Dissolved oxygen was removed by bubbling a nitrogen gas into thesolution while keeping the temperature of the aqueous monomer solutionat 25° C. Next, 52 parts aqueous 10 percent 2,2-azobis (2-methylpropionamidine)dihydrochloride solution was added while agitating theaqueous monomer solution. Thereafter, the aqueous solution was agitatedat 25° C. under the flow of nitrogen.

About 2.5 minutes later from the initiation of agitation, the aqueoussolution appeared cloudy or white, and 2,2′-azobis(2-methylpropionamidine)diacrylate in the form of white fine particles with anaverage particle diameter of 9 μm was generated. The2,2′-azobis(2-methyl propionamidine)diacrylate was evenly dispersed inthe aqueous monomer solution.

At this point, the concentration of the acrylate monomer was dilutedfrom about 42 percent to about 35.5 percent by adding 808 parts water tothe aqueous monomer solution. Thereafter, 306 parts aqueous 10 percentsodium persulfate solution and 1.7 parts aqueous 1 percent L-ascorbicacid solution were added while agitating the aqueous monomer solution toprepare an aqueous monomer solution. After the addition, the aqueousmonomer solution continued to be agitated.

About 30 seconds after the addition of aqueous sodium persulfatesolution, polymerization of the acrylate monomer was initiated. As aresult, a hydrogel having cells was obtained.

The resulting hydrogel having cells was removed, and the same operationsas in Example 1 were performed to produce a water-absorbent resin. Theaverage pore diameter of the water-absorbent resin was 100 μm. Moreover,various physical properties of the water-absorbent resin were measuredby the above-mentioned methods. The water retention capacity was 38 g/g,the residual monomer content was 270 ppm, the water-soluble componentcontent was 9 percent, the dispersion rate was 23 seconds, the dry touchwas 4.1 g, and the absorbent capacity under pressure was 10 g/g. Theresults are shown in Table 2.

EXAMPLE 14

First, by performing the same polymerization as in Example 1, a hydrogelhaving cells was obtained. More specifically, by leaving the aqueousmonomer solution statically to polymerize the acrylate monomer, thehydrogel having cells as a porous cross-linked polymer was produced.

The resulting hydrogel having cells was removed and cut into a sheetwith a thickness of about 5 mm. Thereafter, a microwave was radiated tothe hydrogel having cells using a home-use microwave with a frequency of2,450 MHz (trade name: NE-A460, produced by Matsushita ElectricIndustrial Co., Ltd.). About 30 seconds after the initiation ofradiation, the hydrogel having cells was dried in a state in which itwas swelled to a size about 10 times larger than the size beforeradiated. Thus, a sheet of water-absorbent resin of the presentinvention was produced.

The average pore diameter of the water-absorbent resin was about 500 μm.Moreover, various physical properties of the water-absorbent resin weremeasured by the above-mentioned methods. The water retention capacitywas 31 g/g, the residual monomer content was 170 ppm, the water-solublecomponent content was 10 percent, the dispersion rate was 19 seconds,the dry touch was 4.3 g, and the absorbent capacity under pressure was 9g/g. The results are shown in Table 2. FIG. 6 shows an electronphotomicrograph indicating the profile structure of the sheet ofwater-absorbent resin.

EXAMPLE 15

First, by performing the same polymerization as in Example 1, a hydrogelhaving cells was obtained. More specifically, by leaving the aqueousmonomer solution at rest to polymerize the acrylate monomer, thehydrogel having cells as a porous cross-linked polymer was produced.

The resulting hydrogel having cells was removed and cut into a sheetwith a thickness of about 5 mm. Thereafter, the hydrogel having cellswas hot-air dried at 170° C. using a hot-air drier. The hydrogel havingcells was dried in a state in which it was swelled into a size about 1.5times larger than the original size. Thus, a sheet of water-absorbentresin of the present invention was produced.

The average pore diameter of the water-absorbent resin was 250 μm.Moreover, various physical properties of the water-absorbent resin weremeasured by the above-mentioned methods. The water retention capacitywas 30 g/g, the residual monomer content was 200 ppm, the water-solublecomponent content was 9 percent, the dispersion rate was 24 seconds, thedry touch was 4.5 g, and the absorbent capacity under pressure was 9g/g. The results are shown in Table 2.

EXAMPLE 16

The water-absorbent resin obtained in Example 13 was subjected tosecondary cross-linking treatment. First, 0.05 parts ethylene glycoldiglycidyl ether, 0.75 parts glycerin, 0.5 parts lactic acid, 3 partswater, and 0.75 parts isopropyl alcohol were mixed to prepare a mixedsolution.

Next, 100 parts water-absorbent resin produced in Example 13 and themixed solution were combined, and the resulting mixture was heated at195° C. for 20 minutes . As a result, a water-absorbent resin having acovalent bond and improved cross-link density in the vicinity ofsurface, i.e., a water-absorbent resin which underwent the secondarycross-linking treatment, was obtained. The average pore diameter of thewater-absorbent resin was 600 μm. Moreover, various physical propertiesof the water-absorbent resin were measured by the above-mentionedmethods. The water retention capacity was 35 g/g, the residual monomercontent was 250 ppm, the water-soluble component content was 9 percent,the dispersion rate was 14 seconds, the dry touch was 3.5 g, and theabsorbent capacity under pressure was 33 g/g. The results are shown inTable 2.

COMPARATIVE EXAMPLE 4

First, by performing the same polymerization as in Comparative Example1, a hydrogel was produced. More specifically, by leaving the aqueousmonomer solution at rest to polymerize the acrylate monomer, thehydrogel was produced.

The resulting hydrogel was removed and cut into a sheet with a thicknessof about 5 mm. Thereafter, a microwave was radiated to the hydrogelusing a home-use microwave so as to dry the hydrogel. As a result, asheet of water-absorbent resin was obtained for comparison purposes.However, a discharge phenomenon occurred at a part of the surface of thehydrogel during drying, and the part was burnt black.

This comparative water-absorbent resin had no pore. Moreover, variousphysical properties of the comparative water-absorbent resin weremeasured by the above-mentioned methods. The water retention capacitywas 27 g/g, the residual monomer content was 640 ppm, the water-solublecomponent- content was 15 percent, the dispersion rate was 83 seconds,the dry touch was 7.1 g, and the absorbent capacity under pressure was 6g/g. Thus, the comparative water-absorbent resin had declined dispersionrate and dry touch. The results are shown in Table 2.

COMPARATIVE EXAMPLE 5

An aqueous monomer solution was prepared in the same manner as inExample 11. By bubbling a nitrogen gas into the aqueous monomer solutionwhile keeping the temperature of the aqueous monomer solution at 25° C.,dissolved oxygen was removed. Next, 40 parts aqueous 10 percent2,2′-azobis (2-methyl propionamidine) dihydrochloride solution was addedwhile agitating the aqueous monomer solution. 28 parts aqueous 10percent sodium persulfate solution and 1.3 parts aqueous 1 percentL-ascorbic acid solution were immediately added to the aqueous monomersolution. Thereafter, the acrylate monomer was polymerized by agitatingthe aqueous monomer solution. As a result, a hydrogel having cells wasproduced. Namely, the acrylate monomer was polymerized without using ablowing agent of the present invention. The hydrogel had cells rangingfrom 1 mm to 3 mm in size.

The resulting hydrogel having cells was removed, and the same operationsas in Example 1 were performed to produce a comparative water-absorbentresin. The resulting comparative water-absorbent resin had a particlediameter ranging from 850 μm to 10 μm, and almost no pores. Moreover,various physical properties of the comparative water-absorbent resinwere measured by the above-mentioned methods. The water retentioncapacity was 39 g/g, the residual monomer content was 940 ppm, thewater-soluble component content was 12 percent, the dispersion rate was42 seconds, the dry touch was 6.0 g, and the absorbent capacity underpressure was 8 g/g. Thus, the comparative water-absorbent resin hadincreased residual monomer content, and declined dispersion rate and drytouch. The results are shown in Table 2.

COMPARATIVE EXAMPLE 6

An aqueous monomer solution was prepared in the same manner as inExample 1. By bubbling a nitrogen gas into the aqueous monomer solutionwhile keeping the temperature of the aqueous monomer solution at 25° C.,dissolved oxygen was removed. Next, sorbitan monostearate as a surfaceactive agent was added in an amount of 0.2 percent based on the amountof the aqueous monomer solution, and then cyclohexane as a liquidblowing agent was added in an amount of 23 percent based on the amountof the aqueous monomer solution while agitating the aqueous monomersolution. As a result, cyclohexane having an average particle diameterof about 50 μm was evenly dispersed in the aqueous monomer solution.

Thereafter, 2.6 parts aqueous 10 percent sodium persulfate solution and1 part aqueous 1 percent L-ascorbic acid solution were added whileagitating the aqueous monomer solution. After sufficient agitation, theaqueous monomer solution was kept at rest to polymerize the acrylatemonomer. As a result, a hydrogel was produced. Namely, the acrylatemonomer was polymerized without using a blowing agent of the presentinvention. The hydrogel had cells ranging from 2 mm to 3 mm in size.

The resulting hydrogel was removed, and the same operations as inExample 1 were performed to produce a comparative water-absorbent resin.The resulting comparative water-absorbent resin had particle diametersranging from 850 μm to 10 μm, and almost no pores. Moreover, thecomparative water-absorbent resin had an odor of cyclohexane.

Various physical properties of the comparative water-absorbent resinwere measured by the above-mentioned methods. The water retentioncapacity was 27 g/g, the residual monomer content was 540 ppm, thewater-soluble component content was 12 percent, the dispersion rate was63 seconds, the dry touch was 7.4 g, and the absorbent capacity underpressure was 7 g/g. Thus, the comparative water-absorbent resin haddeclined dispersion rate and dry touch. The results are shown in Table2.

COMPARATIVE EXAMPLE 7

An aqueous monomer solution was prepared in the same manner as inExample 1. By bubbling a nitrogen gas into the aqueous monomer solutionwhile keeping the temperature of the aqueous monomer solution at 25° C.,dissolved oxygen was removed. Next, ethylene carbonate as a liquidblowing agent was dissolved in the aqueous monomer solution whileagitating the aqueous monomer solution so as to be 1 percent based onthe amount of aqueous monomer solution.

Thereafter, 2.6 parts aqueous 10 percent sodium persulfate solution and1 part aqueous 1 percent L-ascorbic acid solution were added whileagitating the aqueous monomer solution. After sufficient agitation, theaqueous monomer solution was kept at rest to polymerize the acrylatemonomer. As a result, a hydrogel was produced. Namely, the acrylatemonomer was polymerized without using a blowing agent of the presentinvention. The hydrogel had cells. ranging from 2 mm to 3 mm in size.

The resulting hydrogel was removed, and the same operations as inExample 1 were performed to produce a comparative water-absorbent resin.The resulting comparative water-absorbent resin had particle diametersranging from 850 μm to 10 μm, and almost no pores.

Various physical properties of the comparative water-absorbent resinwere measured by the above-mentioned methods. The water retentioncapacity was 23 g/g, the residual monomer content was 740 ppm, thewater-soluble component content was 7 percent, the dispersion rate was73 seconds, the dry touch was 8.4 g, and the absorbent capacity underpressure was 9 g/g. Thus, the comparative water-absorbent resin haddeclined dispersion rate and dry touch. The results are shown in Table2.

TABLE 2 Water Water- absorbent Water Residual soluble capacity Averageretention monomer component Dispersion Dry under pore capacity contentcontent rate touch pressure diameter (g/g) (ppm) (%) (second) (g) (g/g)(μm) Examples 10 30  70 9 40 4.5 10  50 11 39 240 8 21 4.0 11 150 12 38270 9 24 4.0 10 100 13 38 270 9 23 4.1 10 100 14 31 170 10  19 4.3  9500 15 30 200 9 24 4.5  9 250 16 35 250 9 14 3.5 33 600 Comparativeexamples  4 27 640 15  83 7.1  6 —  5 39 940 12  42 6.0  8 —  6 27 54012  63 7.4  7 —  7 23 740 7 73 8.4  9 —

EXAMPLE 17

First, by performing the same polymerization as in Example 1, a hydrogelhaving cells was obtained. More specifically, by leaving the aqueousmonomer solution at rest to polymerize the acrylate monomer, thehydrogel having cells as a porous cross-linked polymer was produced.

The resulting hydrogel having cells was removed, cut into pieces in sizeranging from about 20 mm to 1 mm, and then hot-air-dried at 150° C. witha hot-air drier. Thereafter, the dried pieces were ground with a rollmill, and sieved using a 20-mesh screen, thereby obtaining awater-absorbent resin of the present invention.

It was confirmed through an electron photomicrograph that the resultingwater-absorbent resin was porous. The average pore diameter of thewater-absorbent resin was 60 μm. Next, 0.3 parts inorganic powder (tradename: AEROSIL 200, produced by Nippon Aerosil Co., Ltd.) was added to100 parts water-absorbent resin, and sufficiently mixed, therebyproducing a water-absorbent resin composition of the present invention.Various physical properties of the resulting water-absorbent resincomposition were measured by the above-mentioned methods. The waterretention capacity was 33 g/g, the absorption rate was 77 seconds, andthe liquid permeability was 112 seconds. The results are shown in Table3.

EXAMPLE 18

First, by performing the same polymerization as in Example 2, a hydrogelhaving cells was obtained. More specifically, by leaving the aqueousmonomer solution at rest to polymerize the acrylate monomer, thehydrogel having cells as a porous cross-linked polymer was produced.

The resulting hydrogel having cells was removed, cut into pieces in sizeranging from about 20 mm to 1 mm, and then hot-air-dried at 150° C. witha hot-air drier. Thereafter, the dried pieces were ground with a rollmill, and sieved using a 20-mesh screen, thereby obtaining awater-absorbent resin of the present invention.

Next, the water-absorbent resin was subjected to secondary cross-linkingtreatment. First, 1 part glycerin, 3 parts water and 1 part isopropylalcohol were mixed to prepare a mixed solution. Subsequently, 100 partswater-absorbent resin and the mixed solution were combined, and theresulting mixture was heated at 195° C. for 25 minutes. As a result, awater-absorbent resin having a covalent bond and improved cross-linkdensity in the vicinity of surface, i.e., a water-absorbent resin whichunderwent the secondary cross-linking treatment, was obtained. Theaverage pore diameter of the water-absorbent resin was 70 μm. Theresidual monomer content was 150 ppm, the water-soluble componentcontent was 5 percent, and the absorbent capacity under pressure was 31g/g.

Next, 0.3 parts inorganic powder (trade name: AEROSIL 200, produced byNippon Aerosl Co., Ltd.) was added to 100 parts water-absorbent resin,and sufficiently mixed, thereby producing a water-absorbent resincomposition of the present invention. Various physical properties of theresulting water-absorbent resin composition were measured by theabove-mentioned methods. The water retention capacity was 31 g/g, theabsorption rate. was 66 seconds, and the liquid permeability underpressure was 32 seconds. The results are shown in Table 3.

EXAMPLE 19

First, 38.6 parts acrylic acid, 409 parts aqueous 37 percent sodiumacrylate solution, 0.45 parts trimethyloipropane triacrylate, and 53parts deionized water were mixed to prepare an aqueous monomer solution.

By bubbling a nitrogen gas into the aqueous monomer solution whilekeeping the temperature of the aqueous monomer solution at 25° C.,dissolved oxygen was removed. Next, 4.3 parts aqueous 10 percent2,2′-azobis(2-methyl propionamidine)dihydrochloride solution was addedwhile agitating the aqueous monomer solution. Thereafter, the aqueoussolution was agitated at 25° C. under the flow of nitrogen.

About seven minutes later from the initiation of agitation, the aqueoussolution appeared cloudy or white, and 2,2′-azobis(2-methylpropionamidine)diacrylate in the form of white fine solid particles withan average particle diameter of 9 um was generated. Moreover, 11 minuteslater from the initiation of agitation, the amount of 2,2′-azobis(2-methyl propionamidine) diacrylate generated became 0.32 percent basedon the acrylate monomer. The 2,2′-azobis (2-methyl propionamidine)diacrylate was evenly dispersed in the aqueous monomer solution.

At this time, 2.6 parts aqueous 10 percent sodium persulfate solutionand 1 part aqueous 1 percent L-ascorbic acid solution were added whileagitating the aqueous monomer solution. After the addition, the aqueousmonomer solution continued to be agitated.

About 10 minutes after the addition of aqueous sodium persulfatesolution, the temperature of the aqueous monomer solution reached about88° C. Thereafter, the aqueous monomer solution continued to be agitatedfor further 12 minutes while keeping the temperature thereof between 70°C. and 80° C. so as to polymerize the acrylate monomer. As a result, ahydrogel having cells was obtained.

The resulting hydrogel having cells was removed, made into small piecesin size ranging from about 20 mm to 1 mm, and hot-air-dried at 150° C.with a hot-air drier. Subsequently, the dried pieces were ground with aroll mill, and classified using a 20-mesh screen, thereby producing awater-absorbent resin of the present invention.

Next, the water-absorbent resin was subjected to secondary cross-linkingtreatment. First, 0.05 parts ethylene glycol diglycidyl ether, 0.75parts glycerin, 0.5 parts lactic acid, 3 parts water and 0.75 partsisopropyl alcohol were mixed to prepare a mixed solution. Subsequently,100 parts water-absorbent resin and the mixed solution were combined,and the resulting mixture was heated at 195° C. for 20 minutes.Furthermore, 5 parts aqueous 30 percent polyethylene imine solutionhaving an average molecular weight of 70,000 was mixed into the mixedsolution, and the resulting mixed solution was heated. As a result, awater-absorbent resin having a covalent bond and ionic bond, andimproved cross-link density in the vicinity of surface, i.e., awater-absorbent resin which underwent the secondary cross-linkingtreatment, was obtained.

Thereafter, 0.3 parts inorganic powder (trade name: AEROSIL 200,produced by Nippon Aerosil Co., Ltd.) was added to 100 partswater-absorbent resin, and sufficiently mixed, thereby producing awater-absorbent resin composition of the present invention. Variousphysical properties of the resulting water-absorbent resin compositionwere measured by the above-mentioned methods. The water retentioncapacity was 28 g/g, the absorption rate was 56 seconds, and the liquidpermeability was 23 seconds. The results are shown in Table 3.

COMPARATIVE EXAMPLE 8

First, 800 parts acrylic acid, 4 parts tetraallyloxy ethane, and 3,166parts water were placed in a reaction container to prepare an aqueousmonomer solution. By injecting a nitrogen gas into the aqueous monomersolution, dissolved oxygen was removed, and the temperature of theaqueous monomer solution was set at 10° C.

When the dissolved oxygen in the aqueous monomer solution became 1 ppmor less, an aqueous solution formed by dissolving 2.4 parts2,2′-azobisamidinopropane dihydrochloride in 10 parts water, an aqueoussolution produced by dissolving 0.2 parts ascorbic acid in 10 partswater, and an aqueous solution made by diluting 2.29 parts aqueous 35percent hydrogen peroxide solution with 10 parts water were added inthis order as catalysts.

Polymerization was initiated after while from the addition, and thetemperature of the aqueous monomer solution became the maximumtemperatures ranging from about 65° C. to 70° about two hours later,thereby obtaining a hydrogel. Thereafter, the hydrogel was placed andkept in an insulated container for three hours to reduce the residualmonomer content to 1,000 ppm or less.

Then, the hydrogel was removed and cut into small pieces with a mincer.The temperature of the minced hydrogel was about 66° C. Next, 640 partsaqueous 50 percent sodium hydroxide solution was added to the hydrogel.The temperature of the aqueous 50 percent sodium hydroxide solution was38° C. The aqueous solution was agitated while cutting the hydrogel intosmaller pieces in the aqueous solution so as to uniformly performneutralization. The aqueous solution was heated by the neutralization,and its temperature was raised to temperatures ranging from 88° C. to93° C.

Next, an aqueous solution formed by dissolving 2.4 parts ethylene glycoldiglycidyl ether as a cross-linking agent in 50 parts water was added tothe above aqueous solution. The temperature of the ethylene glycoldiglycidyl ether was 24° C. The aqueous solution was agitated whilecutting the hydrogel into smaller pieces in the aqueous solution so thatthe ethylene glycol diglycidyl ether was evenly dispersed to achieve auniform surface cross-linkage.

Then, the hydrogel to which neutralization and surface cross-linkingsteps were applied was removed, and dried at 105° C. using a rotary drumdryer to arrange the water content in the hydrogel to be 10 percent. Asa result, dried pieces in the form of flakes were obtained.Subsequently, the dried pieces were ground and sieved using a 20-meshscreen and a 325-mesh screen, thereby producing a comparativewater-absorbent resin composition.

Various physical properties of the resulting comparative water-absorbentresin composition were measured by the above-mentioned methods. Thewater retention capacity was 31 g/g, the absorption rate was 87 seconds,and the liquid permeability under pressure was 600 seconds. Thus, thecomparative water-absorbent resin composition had declined absorptionrate and liquid permeability. The results are shown in Table 3.

COMPARATIVE EXAMPLE 9

First, 98.9 grams of aqueous 30 percent sodium hydroxide solution wasadded to an aqueous solution formed by diluting 72.1 grams acrylic acidwith 18.0 grams of water while cooling the aqueous solution forneutralization. Thereafter, 10.7 grams of aqueous 2.8 percent potassiumpersulfate solution was added to the aqueous solution, thereby forming auniform solution. As a result, an aqueous monomer solution to which aradical polymerization initiator was added was prepared.

Meanwhile, a 500 ml flask equipped with an agitator, a reflux condenser,a thermometer, a dropping funnel and a nitrogen gas inlet tube was usedas a-reaction container. 283 ml of cyclohexane and 2.2 grams of aqueous25 percent polyoxyethylene dodecyl ether sodium sulfate salt solutionwere placed in the reaction container, and agitated at 300 rpm, therebydispersing the polyoxyethylene dodecyl ether sodium sulfate salt. Afterperforming nitrogen substitution in the flask, the temperature wasraised to 75° C. Moreover, the aqueous monomer solution was placed inthe dropping funnel.

The aqueous monomer solution was dropped in 30 minutes while agitatingthe cyclohexane solution so as to disperse and suspense the aqueousmonomer solution. After dropping the aqueous monomer solution, thecyclohexane solution was agitated at 75° C. for 1.5 hours, and furtheragitated at 80° C. for four hours so as to perform reverse-phasesuspension-polymerization of the acrylate monomer. Duringpolymerization, water in the reaction container was continuously removedby forming an azeotrope with cyclohexane (azeotropic dehydration).

When the amount of water in the reaction container became 30 percent ofthe amount of water added before the polymerization, 0.18 grams ofethylene glycol diglycidyl ether was added to the reaction container,and reacted for 30 minutes.

After the reaction, the cyclohexane solution was filtered, and theresulting hydrogel was subjected to vacuum drying, thereby obtaining88.0 grams of (sodium) acrylate polymer. Next, the polymer was ground,and sieved with a 20-mesh screen, thereby producing a comparativewater-absorbent resin composition.

Various physical properties of the resulting comparative water-absorbentresin composition were measured by the above-mentioned methods. Thewater retention capacity was 30 g/g, the absorption rate was 81 seconds,and the liquid permeability was 670 seconds. Thus, the comparativewater-absorbent resin composition had declined absorption rate andliquid permeability under pressure. The results are shown in Table 3.

COMPARATIVE EXAMPLE 10

By decomposing commercially available paper diapers, water-absorbentresins were removed. The paper diapers used here were Moony Power-Slim(trade name) produced by Uni Charm Corporation (hereinafter referred toas the product A), Doremi (trade name) produced by Shin Oji PaperManufacturing Co., Ltd. (hereinafter referred to as the product B),Merries Pants (trade name) produced by Kao Corporation. (hereinafterreferred to as the product C), and Moony Man (trade name) produced byUni Charm Corporation (hereinafter referred to as the product D) Variousphysical properties of these comparative water-absorbent resins, i.e.,the commercially available products A to D, were measured by theabove-mentioned methods.

In addition, various physical properties of Sanwet IM-5000 (trade name)produced by Sanyo Chemical Industries, Ltd. (hereinafter referred to asthe product E), and Arasorb 720 (trade name) produced by ArakawaChemical industries, Ltd. (hereinafter referred to as the product F)were measured by the above-mentioned methods.

The results are given in Table 3. The products A to F are inferior atleast in the absorption rate or in the liquid permeability underpressure.

TABLE 3 Liquid Water permeability retention Absorption under abilityrate pressure (g/g) (second) (second) Examples 17 33 77 112 18 31 66  3219 28 56  23 Comparative examples  8 30 81 670  9 31 87 not less than600  Comparative example 10 product A 31 43 254 product B 26 33 not lessthan 1000 product C 29 37 not less than 600  product D 38 87 not lessthan 1000 product E 30 48 not less than 600 product F 20  8 512

While particular embodiments or examples of the present invention havebeen described to carry out the invention in the best mode, it will beobvious that the same may be varied in many ways Such variations are notto be regarded as a departure from the spirit and scope of theinvention, and all such modifications as would be obvious to one skilledin the art are intended to be included within the scope of the followingclaims.

APPLICATIONS OF THE INVENTION TO INDUSTRIAL USE

With the use of the above-mentioned methods, it is possible toindustrially produce water-absorbent resins having excellent waterabsorption characteristics, such as dispersion and absorption rate ofaqueous fluid, water retention capacity and dry touch, lowerwater-soluble component content, and lower residual monomer content, inan inexpensive and easy manner.

Moreover, the above-mentioned structures can provide water-absorbentresins and water-absorbent resin compositions which achieve excellentpermeability and dispersion of aqueous fluid under pressure, andimproved absorption rate and water retention capacity without causing agel blocking phenomenon.

The water-absorbent resins and the water-absorbent resin compositionscan be suitably used in absorbent articles in various fields, forexample: sanitary materials (body fluids absorbent articles) such aspaper diapers, sanitary napkins, incontinence pads, wound protectingmaterial and wound healing material; absorbent articles for absorbingurine of pets; materials of construction and building; such as buildingmaterial, water retentive material for soil, waterproof material,packing material, and gel pusule; materials for food, such as dripabsorbing material, freshness retentive material, and heat insulatingmaterial; various industrial articles, such as oil and water separatingmaterial, condensation preventing material, and coagulant; andagricultural and horticultural articles, such as water retentivematerial for plant and soil. Thus, it is possible to provide absorbentarticles exhibiting excellent performances mentioned above.

What is claimed is:
 1. A process for producing a water-absorbent resincomprising the steps of: polymerizing an unsaturated monomer afterdispersing a solid blowing agent having an average particle diameterwithin a range of from 1 μm to 100 μm in an aqueous monomer solutioncontaining said unsaturated monomer and a cross-linking agent; andforming a covalent, bond by treating a vicinity of a surface of saidwater-adsorbent resin with a surface cross-linking agent after saidpolymerizing step; wherein at least one condition selected from thegroup consisting of conditions (a), (b) and (c) is satisfied: (a) saidblowing agent is an acrylic acid salt of an azo compound containing anamino group represented by formula (1)

(b) said blowing agent is an acrylic acid salt of an azo compoundcontaining an amino group represented by formula (2)

(c) the step of polymerizing said unsaturated monomer i, carried outunder the presence of a dispersion stabilizer.
 2. The process forproducing a water-absorbent resin according to claim 1, wherein the stepof polymerizing said unsaturated monomer is carried out while agitatingsaid aqueous monomer solution.
 3. The process for producing awater-absorbent resin according to claim 1, wherein the step ofpolymerizing said unsaturated monomer is carried out without agitatingsaid aqueous monomer solution.
 4. The process for producing awater-absorbent resin according to claim 1, wherein the step ofpolymerizing said unsaturated monomer is carried out .at temperature sranging from 40° C. to 120° C.
 5. The process for producing awater-absorbent resin according to claim 1, wherein said unsaturatedmonomer contains at least one kind of acrylate monomer selected from thegroup consisting of acrylic acids and water-soluble salts of the acrylicacids as a chief constituent.
 6. The process for producing awater-absorbent resin according to claim 5, wherein said water-solublesalt is-at least one kind of salt selected from the group consisting ofsodium salt and potassium salt.
 7. The process for producing awater-absorbent resin according to claim 1, wherein said cross-linkingagent is at least one kind of agent selected from the group consistingof compounds having a plurality of vinyl groups in a molecule, compoundshaving in a molecule at least one vinyl group and at least onefunctional group reactive with a carboxyl group of said unsaturatedmonomer, and compounds having in a molecule a plurality of functionalgroups reactive with said carboxyl group.
 8. The process for producing awater-absorbent resin according to claim 1, wherein said blowing agentis used in an amount ranging from 0.005 weight parts to 25 weight partsbased on 100 weight parts of said unsaturated monomer.
 9. The processfor producing a water-absorbent resin according to claim 1, wherein saidblowing agent is an acrylic acid salt of an azo compound containing anamino group represented by general formula (1)

(wherein X₁ and X₂ independently represent an alkylene group having 1 to4 carbons, R₁, R₂, R₃, R₄, R₅, and R₆ independently represent a hydrogenatom, alkyl group having 1 to 4 carbons, aryl group, allyl group orbenzyl group).
 10. The process for producing a water-absorbent resinaccording to claim 1, wherein said blowing agent is an acrylic acid saltof an azo compound containing an amino group represented by generalformula (2)

(wherein X₃ and X₄ independently represent an alkylene group having 1 to4 carbons, X₅ and X₆ independently represent an alkylene group having 2to 4 carbons, and R₇ and R₈ independently represent a hydrogen atom oralkyl group having 1 to 4 carbons).
 11. The process for producing awater-absorbent resin according to claim 1, wherein said blowing agentis 2,2′-azobis(2-methyl propion amidine) diacrylate.
 12. The process forproducing a water-absorbent resin according to claim 1, furthercomprising the step of precipitating said blowing agent in said aqueousmonomer solution after dissolving a blowing agent precursor in saidaqueous monomer solution prior to the step of polymerizing saidunsaturated monomer.
 13. The process for producing a water-absorbentresin according to claim 12, wherein the step of precipitating saidblowing agent includes reacting said blowing agent precursor with saidunsaturated monomer.
 14. The process for producing a water-absorbentresin according to claim 12, wherein said blowing agent precursor is ahydrochloride of an azo compound containing an amino group.
 15. Theprocess for producing a water-absorbent resin according to claim 1,wherein the step of polymerizing said unsaturated monomer is carried outunder the presence of a dispersion stabilizer.
 16. The process, forproducing a water-absorbent resin according to claim 15, wherein saiddispersion stabilizer is a surface active agent.
 17. The process forproducing a water-absorbent resin according to claim 15, wherein saiddispersion stabilizer is a water-soluble polymer.
 18. The process forproducing a water-absorbent resin according to claim 15, wherein saiddispersion stabilizer is at least one kind of agent selected from thegroup consisting of polyvinyl alcohol, starch and derivatives thereof,and cellulose and derivatives thereof.
 19. The process for producing awater-absorbent resin according to claim 1, further comprising the stepof forming a covalent bond by treating a vicinity of a surface of saidwater-absorbent resin with a surface cross-linking agent after the stepof polymerizing said unsaturated monomer.
 20. The process for producinga water-absorbent resin according to claim 19, wherein said surfacecross-linking agent is a compound having a plurality of functionalgroups capable of forming a covalent bond by reacting with a carboxylgroup of said water-absorbent resin.
 21. The process for producing awater-absorbent resin according to claim 19, wherein said surfacecross-linking agent is at least one kind of agent selected from thegroup consisting of polyhydric alcohol compounds, epoxy compounds,polyamine compounds, condensation products of polyamine compound andhaloepoxy compound, and alkylene carbonate compounds.
 22. The processfor producing a water-absorbent resin according to claim 19, furthercomprising the step of forming an ionic bond by treating the vicinity ofthe surface of said water-absorbent resin with a cationic compound afterthe step of forming the covalent bond.
 23. The process for producing awater-absorbent resin according to claim 22, wherein said cationiccompound is a compound having a plurality of functional groups capableof forming an ionic bond by reacting with a carboxyl group of saidwater-absorbent resin.
 24. The process for producing a water-absorbentresin according to claim 22, wherein said cationic compound is acationic polymer electrolyte and/or salt thereof.
 25. The process forproducing a water-absorbent resin according to claim 1, furthercomprising the step of drying a hydrogel after the step of polymerizingsaid unsaturated monomer.
 26. The process for producing awater-absorbent resin according to claim 25, further comprising the stepof grinding dried hydrogel after the step of drying said hydrogel.